Phragment v0

Summary: Ref T4205.  This is an initial implementation of Phragment.  You can create and browse fragments in the system (but you can't yet view a fragment's patches / history).

Test Plan: Clicked around and created fragments.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4205

Differential Revision: https://secure.phabricator.com/D7726