Add data attributes to thumbnails on the /uploads, /upload_media_assets,
and /media_assets pages. Add a `data-is-posted` attribute for styling
thumbnails based on whether they've already been posted.
* Make thumbnails on the "My Uploads" page show an icon with an image
count when an upload contains multiple files.
* Make the "My Uploads" page show each upload, not each individual file.
If an upload contains multiple files, they're shown grouped together
under a single upload. This does mean that failed or duplicate uploads
will show up on this page now. This is because this page shows each
upload attempt, not each uniquely uploaded file.
