Aggregate Differential notifications

Summary:
Just a bunch of copy-pasta from D2884. I suppose this calls for
a refactoring at some point...

Test Plan:
Make a bunch of updates, some from different users; check
notifications dropdown and list.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3361