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

Conflicts:
- app/models/media_attachment.rb
  Upstream added audio attachment support
- app/serializers/initial_state_serializer.rb
  Upstream added audio attachment support and how mimetypes are returned
- app/serializers/rest/instance_serializer.rb
  Upstream added a few fields
- config/application.rb
  Upstream added a different paperclip transcoder

app/javascript/styles/mastodon/components.scss

@@ -557,6 +557,7 @@
 
     .compose-form__upload-thumbnail {
       border-radius: 4px;
+      background-color: $base-shadow-color;
       background-position: center;
       background-size: cover;
       background-repeat: no-repeat;