Support "only the first time" in Maniphest

Summary:
Ref T4403. Implements "only the first time" for Maniphest rules, and fixes the trigger itself.

The trigger would never fire and block rules because it was comparing a string (like "first") to an int (like 0).

The "only" vs "every" stuff is contributed and I should have pushed back harder on this toInt / toString stuff. Maybe I'll just get rid of it; it purely causes confusion and problems.

Test Plan: Wrote an "only the first time" rule, ran it twice, it applied once.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4403

Differential Revision: https://secure.phabricator.com/D8193