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