media assets: add dynamically-generated thumbnails (owner-only).

Add ability to dynamically generate thumbnails with:

* https://danbooru.donmai.us/media_assets/6961761.jpg?width=180&height=180

This is currently restricted to the Owner-level user because it's slow.

config/initializers/mime_types.rb

@@ -3,3 +3,7 @@
 # Add new mime types for use in respond_to blocks:
 # Mime::Type.register "text/richtext", :rtf
 Mime::Type.register_alias "application/xml", :sitemap
+
+# XXX remove after upgrading to rack 3.0.0.
+Mime::Type.register "image/webp", :webp
+Mime::Type.register "image/avif", :avif