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.

Test Plan: See screenshots.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5359

Differential Revision: https://secure.phabricator.com/D9548