Add a thumbnail view to the /media_assets page. This page lets you see
all images uploaded to Danbooru by all users (although you can't see who
the uploader is). Also add a link to this page in the subnav bar on the
upload page.
Add a view component for rendering thumbnails for media assets.
This lets us properly show thumbnails on the upload listing page and the
media assets listing page, including support for high pixel density thumbnails
and video length icons for videos.
Fixes not being able to see thumbnails on the /media_assets page.
This is mostly copy/pasted from the post preview component. FIXME: don't duplicate code.
