Allow keyboard navigation between individual changes

Summary:
Permit "j" and "k" to cycle through individual changeblocks, similar to how this
feature works in ReviewBoard. This still needs a bunch of refinement but it's
getting closer to being useful.

Also moved reticle underneath the table so you can click links through it (derp
derp).

Test Plan:
Used "j" and "k" to cycle through individual changes.

Reviewed By: aran
Reviewers: aran, jungejason, tuomaspelkonen
CC: moskov, aran, epriestley
Differential Revision: 426