Improve thumbnails in pholio mock view

Summary:
  - Use "preview" instead of "thumb" so we don't get a white background.
  - Give this element a darker gutter to separate it a little bit visually.
  - Put every thumb on a square hit target.
  - Add some color/border/hover stuff.
  - Ship down image dimensions.
  - Reduce thumb size to 140 so we can fit 2-up on mobile when we get there.

Test Plan:
Before:

{F33545}

After:

{F33546}

Reviewers: chad, ljalonen

Reviewed By: ljalonen

CC: aran

Differential Revision: https://secure.phabricator.com/D5070