Cache the Releeph project edit page's help remarkup

Summary: I wrote some of the Releeph-project edit-page's help in remarkup.  This renders the remarkup using the `PhabricatorMarkupEngine` pipeline.

Test Plan:
* Edit a Releeph project.
* Feel the cool base of a laptop whose CPU is no longer being thrashed by onerous remarkup rendering duties.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3098

Differential Revision: https://secure.phabricator.com/D5898