Show coverage information in Differential

Summary:
Render coverage information in the right gutter, if available.

We could render some kind of summary report deal too but this seems like a good
start.

Test Plan:

• Looked at diffs with coverage.
• Looked at diffs without coverage.
• Used inline comments, diff-of-diff, "show more", "show entire file", "show

generated file", "undo". Nothing seemed disrupted by the addition of a 5th
column.

Reviewers: btrahan, tuomaspelkonen, jungejason

Reviewed By: btrahan

CC: zeeg, aran, epriestley

Maniphest Tasks: T140

Differential Revision: https://secure.phabricator.com/D1527