Version explorer UI

[bctcvai]

Piggy backing off of https://github.com/cvisionai/tator/issues/1567 -- extend this to compare versions themselves. This should allow someone to see the top-level differences between version A and version B. There are a couple of ways of looking at differences in git that are relevant here:

git diff --stat (Nice per-file summary) git diff (Line by line deltas)

I think a lot of mileage could be done by emulating git diff --stat before the visual complexities of the equivalent per-line diff is attempted to be executed.