Make media assets show a placeholder thumbnail when the image is
missing. This can happen if the upload is still processing, or if the
media asset's image was expunged, or if the asset failed during upload
(usually because of some temporary network failure when trying to
distribute thumbnails to the backend image servers).
Fixes a problem where new images on the My Uploads or All Uploads pages
could have broken thumbnails if they were still in the uploading phase.
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.
