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/controllers/admin/base_controller.rb

@@ -20,5 +20,9 @@ module Admin
     def set_pack
       use_pack 'admin'
     end
+
+    def set_user
+      @user = Account.find(params[:account_id]).user || raise(ActiveRecord::RecordNotFound)
+    end
   end
 end