Merge branch 'master' into glitch-soc/merge-upstream

Conflicts:
- app/controllers/admin/base_controller.rb
  Some refactoring made upstream, no real conflict.
- app/javascript/mastodon/features/compose/components/compose_form.js
  Updated using upstream's code but using maxChars instead of the
  hardcoded length of 500 characters per toot.
- app/javascript/styles/mastodon/components.scss
  Upstream redesigned the onboarding modal. Not sure why we had a
  conflict there.

Gemfile

@@ -40,7 +40,7 @@ end
 gem 'net-ldap', '~> 0.10'
 gem 'omniauth-cas', '~> 1.1'
 gem 'omniauth-saml', '~> 1.10'
-gem 'omniauth', '~> 1.2'
+gem 'omniauth', '~> 1.9'
 
 gem 'doorkeeper', '~> 5.0'
 gem 'fast_blank', '~> 1.0'
@@ -58,7 +58,7 @@ gem 'idn-ruby', require: 'idn'
 gem 'kaminari', '~> 1.1'
 gem 'link_header', '~> 0.0'
 gem 'mime-types', '~> 3.2', require: 'mime/types/columnar'
-gem 'nokogiri', '~> 1.8'
+gem 'nokogiri', '~> 1.9'
 gem 'nsa', '~> 0.2'
 gem 'oj', '~> 3.7'
 gem 'ostatus2', '~> 2.0'