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/controllers/api/v1/media_controller.rb

@@ -39,7 +39,7 @@ class Api::V1::MediaController < Api::BaseController
   end
 
   def media_attachment_params
-    params.permit(:file, :description, :focus)
+    params.permit(:file, :thumbnail, :description, :focus)
   end
 
   def file_type_error