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

Conflicts:
- `app/javascript/mastodon/actions/compose.js`:
  Not a “real” conflict, but change too close to a change we made to
  fix the vanilla WebUI locally pushing authored local-only toots in the
  public TL view.

app/serializers/rest/media_attachment_serializer.rb

@@ -4,7 +4,7 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :id, :type, :url, :preview_url,
-             :remote_url, :text_url, :meta,
+             :remote_url, :preview_remote_url, :text_url, :meta,
              :description, :blurhash
 
   def id
@@ -35,6 +35,10 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
     end
   end
 
+  def preview_remote_url
+    object.thumbnail_remote_url.presence
+  end
+
   def text_url
     object.local? ? medium_url(object) : nil
   end