Fancier tooltips for revision lists

Summary:
Put flag note in a Javelin tooltip on the flag, and extra
reviewers in a Javelin tooltip on the (+N). This is a bit more expensive
because we have to fetch all of their usernames but I'm hoping that's
okay?

Test Plan:
Load a bunch of revision lists. Mouse on. Mouse off. Mouse
on. Mouse off.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3187