R1:53f527f71fb3

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…