* Allow hiding of reblogs from followed users

This adds a new entry to the account menu to allow users to hide
future reblogs from a user (and then if they've done that, to show
future reblogs instead).

This does not remove or add historical reblogs from/to the user's
timeline; it only affects new statuses.

The API for this operates by sending a "reblogs" key to the follow
endpoint. If this is sent when starting a new follow, it will be
respected from the beginning of the follow relationship (even if
the follow request must be approved by the followee). If this is
sent when a follow relationship already exists, it will simply
update the existing follow relationship. As with the notification
muting, this will now return an object ({reblogs: [true|false]}) or
false for each follow relationship when requesting relationship
information for an account. This should cause few issues due to an
object being truthy in many languages, but some modifications may
need to be made in pickier languages.

Database changes: adds a show_reblogs column (default true,
non-nullable) to the follows and follow_requests tables. Because
these are non-nullable, we use the existing MigrationHelpers to
perform this change without locking those tables, although the
tables are likely to be small anyway.

Tests included.

See also <https://github.com/glitch-soc/mastodon/pull/212>.

* Rubocop fixes

* Code review changes

* Test fixes

This patchset closes #648 and resolves #3271.

* Rubocop fix

* Revert reblogs defaulting in argument, fix tests

It turns out we needed this for the same reason we needed it in muting:
if nil gets passed in somehow (most usually by an API client not passing
any value), we need to detect and handle it.

We could specify a default in the parameter and then also catch nil, but
there's no great reason to duplicate the default value.

* Add structure for lists

* Add list timeline streaming API

* Add list APIs, bind list-account relation to follow relation

* Add API for adding/removing accounts from lists

* Add pagination to lists API

* Add pagination to list accounts API

* Adjust scopes for new APIs

- Creating and modifying lists merely requires "write" scope
- Fetching information about lists merely requires "read" scope

* Add test for wrong user context on list timeline

* Clean up tests

All the migrations have been updated to use BIGINTs for ID fields in the DB, but ActiveRecord needs to be told to treat those values as BIGINT as well. This PR does that.

* Make IdsToBigints (mostly!) non-blocking

This pulls in GitLab's MigrationHelpers, which include code to make
column changes in ways that Postgres can do without locking. In general,
this involves creating a new column, adding an index and any foreign
keys as appropriate, adding a trigger to keep it populated alongside
the old column, and then progressively copying data over to the new
column, before removing the old column and replacing it with the new
one.

A few changes to GitLab's MigrationHelpers were necessary:

* Some changes were made to remove dependencies on other GitLab code.
* We explicitly wait for index creation before forging ahead on column
  replacements.
* We use different temporary column names, to avoid running into index
  name length limits.
* We rename the generated indices back to what they "should" be after
  replacing columns.
* We rename the generated foreign keys to use the new column names when
  we had to create them. (This allows the migration to be rolled back
  without incident.)

# Big Scary Warning

There are two things here that may trip up large instances:

1. The change for tables' "id" columns is not concurrent. In
   particular, the stream_entries table may be big, and does not
   concurrently migrate its id column. (On the other hand, x_id type
   columns are all concurrent.)
2. This migration will take a long time to run, *but it should not
   lock tables during that time* (with the exception of the "id"
   columns as described above). That means this should probably be run
   in `screen` or some other session that can be run for a long time.
   Notably, the migration will take *longer* than it would without
   these changes, but the website will still be responsive during that
   time.

These changes were tested on a relatively large statuses table (256k
entries), and the service remained responsive during the migration.
Migrations both forward and backward were tested.

* Rubocop fixes

* MigrationHelpers: Support ID columns in some cases

This doesn't work in cases where the ID column is referred to as a
foreign key by another table.

* MigrationHelpers: support foreign keys for ID cols

Note that this does not yet support foreign keys on non-primary-key
columns, but Mastodon also doesn't yet have any that we've needed to
migrate.

This means we can perform fully "concurrent" migrations to change ID
column types, and the IdsToBigints migration can happen with effectively
no downtime. (A few operations require a transaction, such as renaming
columns or deleting them, but these transactions should not block for
noticeable amounts of time.)

The algorithm for generating foreign key names has changed with this,
and therefore all of those changed in schema.rb.

* Provide status, allow for interruptions

The MigrationHelpers now allow restarting the rename of a column if it
was interrupted, by removing the old "new column" and re-starting the
process.

Along with this, they now provide status updates on the changes which
are happening, as well as indications about when the changes can be
safely interrupted (when there are at least 10 seconds estimated to be
left before copying data is complete).

The IdsToBigints migration now also sorts the columns it migrates by
size, starting with the largest tables. This should provide
administrators a worst-case scenario estimate for the length of
migrations: each successive change will get faster, giving admins a
chance to abort early on if they need to run the migration later. The
idea is that this does not force them to try to time interruptions
between smaller migrations.

* Fix column sorting in IdsToBigints

Not a significant change, but it impacts the order of columns in the
database and db/schema.rb.

* Actually pause before IdsToBigints

Introduce recent to Follow, as Account and other models have.
This change also adds specs for the scope and the dependents.

* add annotate to Gemfile

* rails g annotate:install

* configure annotate_models

* add schema info to models

* fix rubocop to add frozen_string_literal

StreamEntry model. Simply render Salmon slaps as they are needed

and atom feed generation to some extent, as well as the way mentions are
stored