Service "Group by: Project" in Maniphest out of a local index

Summary:
See discussion in D6955. Currently, the logic for "Group by: Project" is roughly:

• Load every possible result.
• Lots of in-process garbage.

Instead, use the new local project name index (from D6957) to service this query more reasonably. Basically:

• Join a table which has keyed project names.
• Order by that table.

Test Plan: {F58033}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6958