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