R1:dcf3b4d32299

Reduce Pholio brokenness for non-image files

Summary:
Ref T5359. When users upload non-image file types (PDFs, text files, whatever), Pholio currently chokes in a few places. Make most of these behaviors more reasonable:

- Provide thumbs in the required sizes.
- Predict the thumb size of these files correctly.
- Disable inline comments.
- Make "View Fullsize" and "Download" into buttons. These mostly-work. Download should probaly really download, but CSRF on forms is a bit of a pain right now.