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/media_proxy_controller.rb

@@ -28,8 +28,8 @@ class MediaProxyController < ApplicationController
   private
 
   def redownload!
-    @media_attachment.file_remote_url = @media_attachment.remote_url
-    @media_attachment.created_at      = Time.now.utc
+    @media_attachment.download_file!
+    @media_attachment.created_at = Time.now.utc
     @media_attachment.save!
   end