R1:17306b7a92bb

Provide basic keyboard navigation support for Differential.

Summary:
ReviewBoard has a fancier version of this feature that's more granular -- the
keyboard can focus on individual changes. I think that's good and intend to
implement something similar, but this gets us a step closer and gets rid of some
of the bookkeeping stuff like making shortcuts discoverable.

(I have another brnach with Maniphest merging which also uses fatcow icons,
which is why the README seems a little out of context.)

Test Plan…