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/javascript/mastodon/features/status/components/detailed_status.js

@@ -125,7 +125,8 @@ class DetailedStatus extends ImmutablePureComponent {
             src={attachment.get('url')}
             alt={attachment.get('description')}
             duration={attachment.getIn(['meta', 'original', 'duration'], 0)}
-            poster={status.getIn(['account', 'avatar_static'])}
+            poster={attachment.get('preview_url') || status.getIn(['account', 'avatar_static'])}
+            blurhash={attachment.get('blurhash')}
             height={150}
           />
         );