Add customizable thumbnails for audio and video attachments (#14145)

- Change audio files to not be stripped of metadata
- Automatically extract cover art from audio if it exists
- Add `thumbnail` parameter to `POST /api/v1/media`, `POST /api/v2/media` and `PUT /api/v1/media/:id`
- Add `icon` to represent it in attachments in ActivityPub
- Fix `preview_url` containing URL of missing missing image when there is no thumbnail instead of null
- Fix duration of audio not being displayed on public pages until the file is loaded

app/serializers/rest/media_attachment_serializer.rb

@@ -28,7 +28,9 @@ class REST::MediaAttachmentSerializer < ActiveModel::Serializer
   def preview_url
     if object.needs_redownload?
       media_proxy_url(object.id, :small)
-    else
+    elsif object.thumbnail.present?
+      full_asset_url(object.thumbnail.url(:original))
+    elsif object.file.styles.key?(:small)
       full_asset_url(object.file.url(:small))
     end
   end