Hovercard tweaks

Summary: Tightens up spacing, remove some of the borders, add alpha channel, make them all blue (sorry, red green and yellow are for 'status'). If we want to do more colors just for hovercards, I have a brown and a black in the mock, but would like to try just blue for now.

Test Plan: UIExamples, Tasks, People, Diffs, and Pastes.

Reviewers: epriestley, AnhNhan, btrahan

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5609