Support basic notification aggregation

Summary: This is both only partially complete (supports Maniphest only) and somewhat overcomplicated (includes support for applying similar algorithms to Feed), but provides runtime aggregation of notifications.

Test Plan: {F13502}

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2884