Restore coverage reporting to Diffusion browse UI


Restore coverage reporting to Diffusion browse UI

Depends on D19377. Ref T13125. Ref T13124. Ref T13105. Coverage reporting in Diffusion didn't initially survive the transition to Document Engine; restore it.

This adds some tentative/theoretical support for multiple columns of coverage, but no way to actually produce them in the UI. For now, the labels, codes, and colors are hard coded.

Test Plan:
Added coverage with diffusion.updatecoverage, saw coverage in the UI:

Screen Shot 2018-04-17 at 6.43.40 AM.png (1×1 px, 231 KB)

Hovered over coverage, got labels and highlighting.

Double-checked labels for "N" (Not Executable) and "U" (Uncovered). See PHI577.

Faked some multi-column coverage, but you can't currently get this yourself today:

Screen Shot 2018-04-17 at 6.16.40 AM.png (1×1 px, 242 KB)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13125, T13124, T13105

Differential Revision: https://secure.phabricator.com/D19378