First pass at hovercard design

Summary:
Refs T1048 - I'm pretty happy and happy to tell you

Savepoint CR

Everything's dummy right now. If you are testing locally, don't forget to edit the PHIDs. Or populate your own handles. Doesn't really matter.

I'm mainly sending it in for the CSS, not the messy PHP code. Ignore the `margin: auto`, that's just for looking nice.

Test Plan: UI Example » Hovercard

Reviewers: epriestley, chad

CC: aran, Korvin

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5519