Clean up initialization of Differential Show More Behavior in Maniphest

Summary:
add a static variable to the method and use it so we don't init more
than once!

Test Plan:
add a "phlog" and noted only init'd one time. verified "show more"
links worked correctly.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T666

Differential Revision: https://secure.phabricator.com/D1553