Improve display of "cancel" button in the cover art uploader

The "cancel" button in the cover art uploader should be styled consistent with the rest of the site and with the negative class (like I did in https://github.com/96187/musicbrainz-server/commit/0d3923b6da94e0ee4791420c3f13f20d5e493377 )

I also think it should be "Remove" and not "Cancel" - you have a list of files and you're removing one from the list, so "cancel" doesn't really work for me there.