Merge with upstream's 1.6.0

Merge with 1.6.0rc4

STRAP IN BUCKAWOO HERE WE GO AGAIN

I translated the additional text ( added by #4871)

When a new user confirms their e-mail, bootstrap their home timeline
by automatically following a set of accounts. By default, all local
admin accounts (that are unlocked). Can be customized by new admin
setting (comma-separated usernames, local and unlocked only)

Conflicts:
      app/javascript/mastodon/features/getting_started/index.js
      app/javascript/packs/public.js
      app/javascript/styles/components.scss

Sync with upstream @ v1.6.0rc3

ohhhhhhhhhhh heck here we go

Conflicts:
	app/javascript/mastodon/components/status_list.js
	app/javascript/mastodon/features/notifications/index.js
	app/javascript/mastodon/features/ui/components/modal_root.js
	app/javascript/mastodon/features/ui/components/onboarding_modal.js
	app/javascript/mastodon/features/ui/index.js
	app/javascript/styles/about.scss
	app/javascript/styles/accounts.scss
	app/javascript/styles/components.scss
	app/presenters/instance_presenter.rb
	app/services/post_status_service.rb
	app/services/reblog_service.rb
	app/views/about/more.html.haml
	app/views/about/show.html.haml
	app/views/accounts/_header.html.haml
	config/webpack/loaders/babel.js
	spec/controllers/api/v1/accounts/credentials_controller_spec.rb

* wrong preposition + typo

* wrong preposition + typo

* Typo

* Typo

* minor changes

* minor changes

It wasn't working ideally and introduced some annoying false positivies

Resolved:

* Lot of redundant renders while mouse moving
* Scroll jumping when timeline loaded
* Scroll position isn't kept when statuses below the scrollTop was deleted then new status arrived

Unresolved:

* Scroll position isn't kept when statuses over the scrollTop was deleted then new status arrived
-> It needs to know which statuses are over the scrollTop
* New status indicator should be active when new statuses arrived while mouse moved recently
-> It needs a) update indicator in ScrollableList, or b) set scrollTop status while mouse moving

* Scrollable tables in settings pages

* Add space before curly brace

To reflect status posting immediately, we've inserted the status into timelines directly. However, status insertion changes "latest status", and it means next timeline refresh only fetches statuses since the inserted status. This behavior is very bad for disconnected timeline and mobile views.

After this patch, it refreshes timeline for disconnected timelines, instead of direct insertion.

In before, the method uses stream_entry id as status id, so replied status was wrongly selected.

This PR uses StatusFinder which was introduced with `Api::Web::EmbedsController`.

* Missing "navigation_bar.pins"

* Missing "navigation_bar.pins"

* Fix language filter codes

CLD3 returns BCP-47 language identifier, filter settings expect
identifiers in the ISO 639-1 format. Convert between formats,
and exclude duplicate languages from filter choices (zh-CN->zh)

* Fix zh name