Make the quick create menu more or less work correctly

Summary:
Ref T3623. I'm sure I didn't get the margins / drop shadow quite right, but this looks and works reasonably well:

{F105637}

Test Plan: Clicked stuff to quick create.

Reviewers: chad, btrahan

Reviewed By: chad

CC: chad, aran

Maniphest Tasks: T3623

Differential Revision: https://secure.phabricator.com/D8089