Add Phriction to the main nav menu

Summary:
A few minor things:

• Add Phriction to the homepage.
• Link to remarkup reference on the edit screen.
• Add a help tab with a super basic help document.

Test Plan: Cursory inspection.
Reviewed By: codeblock
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, epriestley, codeblock
Differential Revision: 655