Restore coverage reporting to Diffusion browse UI
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.
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:
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D19378