R1:b537d1ec01fb

Show a tooltip about code coverage

Summary:
- Show a tip in the margin about what coverage colors mean.
- Highlight the line when mousing over coverage.
- Randomly change the colors to different colors.
- Fix a bug with "show more" that I introduced with the other coverage diff (oops!)

Test Plan: Moused over coverage things.

Reviewers: tuomaspelkonen, btrahan

Reviewed By: tuomaspelkonen

CC: aran, epriestley

Maniphest Tasks: T965

Differential Revision: https://secure.phabricator.com/D1874