R1:ac09345083e9

Add a basic tooltip UI element

Summary: There are a few things we can improve with tooltips.

Test Plan: Moused over all the stuff on the test page.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T965

Differential Revision: https://secure.phabricator.com/D1870