Sort milestones by milestone number, not ID

Summary: Ref T10350. Normally, milestone numbers and IDs have the same order, but they may not if you used the script in T10350 to artificially move a bunch of stuff around.

Test Plan: Milestones now go "1, 2, <thing I artifically moved into position 3>" on local install.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10350

Differential Revision: https://secure.phabricator.com/D15266