Turn thumbs into a history grid thing

Summary:
This could probably use some refinement (and, like, explanatory text, and stronger cues about what rows and columns mean) but feels fairly good to me, at least on test data.

I didn't do any scrolling for now since we have to do full height on mobile anyway I think. I did swap it so the newer ones are on top.

Left/right navigate you among current images only, but you can click any thumb to review history.

Removed history view since it's no longer useful.

Some things that would probably help:

• Some kind of header explaining what this is ("Mock History" or something).
• Stronger visual cue that columns are related by being the same image.
• Clearer cues about obsolete/deleted images (e.g., on the stage itself?)
• Maybe general tweaks.
• Maybe a placeholder (like a grey "X") for images which have been deleted.

(I'm planning to add comment counts too, which I think will be pretty useful, but that felt good to put in another diff.)

Test Plan: See screenshots.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9543