Fix performance issue with large changesets in Differential

Summary:
Fixes T3151. Javelin treats a behavior without parameters as a global behavior and invokes it only once no matter how many times it is initialized (this is necessarily correct for any reasonable behavior, as the inputs do not vary). A recent patch changed `differential-dropdown-menus` from a zero-argument global behavior to an implicitly nonzero-argument behavior by adding `pht`.

Currently, we initialize the behavior next to dropdown menu creation, so this resulted in `O(N^2)` initializations of the menus. For large diffs, this locks browsers. Instead, initialize outside of the dropdown loop so we ginitialize each menu just once.

Test Plan: Viewed a 2,000 file diff without browser lock.

Reviewers: wez, vrana, btrahan

Reviewed By: wez

CC: aran

Maniphest Tasks: T3151

Differential Revision: https://secure.phabricator.com/D5885