Add Z-index to notifications

Summary:
Currently, notifications aren't z-indexed so they can end up underneath some other elements:

{F13144}

Fix this so they float correctly.

Test Plan: {F13166}

Reviewers: jungejason, btrahan

Reviewed By: jungejason

CC: aran

Maniphest Tasks: T1398

Differential Revision: https://secure.phabricator.com/D2834