Add some polish to file attachment switching.
Review Request #229 - Created Dec. 19, 2016 and updated
This change does a few small things to polish up the next/previous file attachments feature: - Instead of just relying on `FileAttachment.__str__`, we use the `display_name` property for the caption. This will fall back on the filename if no caption has been set. - The style for the next/previous switchers had a huge font size set, which looked kind of goofy and didn't match what we had in the review request box. - We have a "dummy" file review UI for file attachments that aren't reviewable. This had some code to set the caption in the review UI header in the case where we were looking at a diff, but didn't set the caption when there was only a single revision. This dummy review UI is still pretty dumb, and should probably have some indication that the file can't be displayed inline, along with a download link, but that can come in some later change. Testing done: Switched around file attachments and saw that things looked better. Reviewed at https://reviews.reviewboard.org/r/8574/