ProjectGroupMapping renamed back to PhabricatorGroupMapping

Summary:
The mappings are actually {projects, keywords} → groups.

This is a partal revert of 77b457cdb9f630ae7479806471fcff7eacb0f4f6.

Test Plan: Run tests

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D286