R1:4c44c44964c4

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