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/workers/redownload_media_worker.rb

@@ -11,7 +11,8 @@ class RedownloadMediaWorker
 
     return if media_attachment.remote_url.blank?
 
-    media_attachment.file_remote_url = media_attachment.remote_url
+    media_attachment.download_file!
+    media_attachment.download_thumbnail!
     media_attachment.save
   rescue ActiveRecord::RecordNotFound
     true