Restore coverage reporting to Diffusion browse UI

Summary:
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:

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:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13125, T13124, T13105

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