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/services/post_status_service.rb

@@ -110,6 +110,7 @@ class PostStatusService < BaseService
     @media = @account.media_attachments.where(status_id: nil).where(id: @options[:media_ids].take(4).map(&:to_i))
 
     raise Mastodon::ValidationError, I18n.t('media_attachments.validations.images_and_video') if @media.size > 1 && @media.find(&:audio_or_video?)
+    raise Mastodon::ValidationError, I18n.t('media_attachments.validations.not_ready') if @media.any?(&:not_processed?)
   end
 
   def language_from_option(str)