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

Conflicts:
- `README.md`:
  Our README.md files are completely different. Discarded upstream changes.
- `app/javascript/core/admin.js`:
  Updating rails-ujs, no real conflict, but a comment to close to changed
  code. Various glitch-soc-only files have been updated to match those changes,
  though.
- `package.json`:
  No real conflict, just an additional dependency in glitch-soc that was too
  close to something updated upstream. Took upstream's changes.

app/serializers/rest/media_attachment_serializer.rb

@@ -12,7 +12,9 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
   end
 
   def url
-    if object.needs_redownload?
+    if object.not_processed?
+      nil
+    elsif object.needs_redownload?
       media_proxy_url(object.id, :original)
     else
       full_asset_url(object.file.url(:original))