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

db/migrate/20200627125810_add_thumbnail_columns_to_media_attachments.rb

@@ -0,0 +1,11 @@
+class AddThumbnailColumnsToMediaAttachments < ActiveRecord::Migration[5.2]
+  def up
+    add_attachment :media_attachments, :thumbnail
+    add_column :media_attachments, :thumbnail_remote_url, :string
+  end
+
+  def down
+    remove_attachment :media_attachments, :thumbnail
+    remove_column :media_attachments, :thumbnail_remote_url
+  end
+end