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