Show other open revisions affecting the same files in Differential

Summary:
I think this feature is probably good, but Differential is also really starting to get a lot of stuff which is not the diff in it. Not sure how best to deal with that.

The mixed table styles are also pretty ugly.

So I guess this is more feedback / proof-of-concept, I think I want to try to improve it somehow before I land it.

Test Plan: Looked at some diffs, some had an awkward, ugly list of diffs affecting the same files.

Reviewers: bill, aran, btrahan

Reviewed By: aran

CC: aran, epriestley

Maniphest Tasks: T829

Differential Revision: https://secure.phabricator.com/D2027