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

lib/paperclip/attachment_extensions.rb

@@ -7,7 +7,7 @@ module Paperclip
     # usage, and we still want to generate thumbnails straight
     # away, it's the only style we need to exclude
     def process_style?(style_name, style_args)
-      if style_name == :original && instance.respond_to?(:delay_processing?) && instance.delay_processing?
+      if style_name == :original && instance.respond_to?(:delay_processing_for_attachment?) && instance.delay_processing_for_attachment?(name)
         false
       else
         style_args.empty? || style_args.include?(style_name)