Fix sorting algorithm for group-by-project

Summary:
When viewing Maniphest tasks grouped by project, there's this
weird algorithm that involves generating strings to use as sort keys.
It's pretty clearly wrong in several cases; this aims to fix it.

Test Plan:
Open Maniphest and try to sort by things. Unfortunately, I
don't have access to a decent Maniphest database, so I'm not certain it
works as it should.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, mikaaay

Maniphest Tasks: T1595

Differential Revision: https://secure.phabricator.com/D3142