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.

app/services/fetch_link_card_service.rb

@@ -137,7 +137,8 @@ class FetchLinkCardService < BaseService
     detector.strip_tags = true
 
     guess      = detector.detect(@html, @html_charset)
-    page       = Nokogiri::HTML(@html, nil, guess&.fetch(:encoding, nil))
+    encoding   = guess&.fetch(:confidence, 0).to_i > 60 ? guess&.fetch(:encoding, nil) : nil
+    page       = Nokogiri::HTML(@html, nil, encoding)
     player_url = meta_property(page, 'twitter:player')
 
     if player_url && !bad_url?(Addressable::URI.parse(player_url))