* cleanup pass

* undo mistakes

* fixed.

* revert

CSFR-prevention is already implemented but adding this doesn't hurt.

A brief introduction to Same-Site cookies (and the difference between strict and
lax) can be found at
https://blog.mozilla.org/security/2018/04/24/same-site-cookies-in-firefox-60/

TLDR: We use lax since we want the cookies to be sent when the user navigates
safely from an external site.

* No need to re-require sidekiq plugins, they are required via Gemfile

* Add derailed_benchmarks tool, no need to require TTY gems in Gemfile

* Replace ruby-oembed with FetchOEmbedService

Reduce startup by 45382 allocated objects

* Remove preloaded JSON-LD in favour of caching HTTP responses

Reduce boot RAM by about 6 MiB

* Fix tests

* Fix test suite by stubbing out JSON-LD contexts

The cache store is explicitly used by some specs, but they were not
isolated and therefore not reliable. This fixes the issue by clearing
the cache after each specs.

* Fix #4058 - Use a long-lived cookie to keep track of user-level sessions

* Fix tests, smooth migrate from previous session-based identifier

* Add overview of active sessions

* Better display of browser/platform name

* Improve how browser information is stored and displayed for sessions overview

* Fix test

* feat: Revocable sessions

* fix: Tests using sign_in

* feat: Configuration entry for the maximum number of session activations

* Dont use raise_error by itself (avoids warning)

* Add coverage for AccountFilter

* Improve coverage and refactor for Subscription#lease_seconds

* Improve coverage and refactor for NotificationMailer

* Simplify assignment of min/max threshold on subscription

* Make private toots get PuSHed to subscription URLs that belong to domains where you have approved followers

* Authorized followers controller, stub for bulk action

* Soft block in the background

* Add simple test for new controller

* Rename Settings::FollowersController to Settings::FollowerDomainsController, paginate results,
rename "private" post setting to "followers-only", fix pagination style, improve post privacy
preferences style, improve warning style

* Extract compose form warnings into own container, show warning when posting to followers-only with unlocked account

Devise changed their approach -
3f3ec236bb

This change silences a deprecation warning about `TestHelpers`

This commit introduces Capybara and the first feature spec.

I focused on coverage for log in for the first feature spec because that would
have prevented 624a9a7136 causing #1236.

behaviour with Turbolinks enabled, optimizing Rabl for production