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/activitypub/note_serializer.rb

@@ -167,6 +167,8 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
     attributes :type, :media_type, :url, :name, :blurhash
     attribute :focal_point, if: :focal_point?
 
+    has_one :icon, serializer: ActivityPub::ImageSerializer, if: :thumbnail?
+
     def type
       'Document'
     end
@@ -190,6 +192,14 @@ class ActivityPub::NoteSerializer < ActivityPub::Serializer
     def focal_point
       [object.file.meta['focus']['x'], object.file.meta['focus']['y']]
     end
+
+    def icon
+      object.thumbnail
+    end
+
+    def thumbnail?
+      object.thumbnail.present?
+    end
   end
 
   class MentionSerializer < ActivityPub::Serializer