R1:a9000ea21cb4

Phriction - lock down /project/ wiki docs

Summary:
only show the blank, "create new" wiki page for the project if the project actually exists; only allow edit if the project actually exists.
Small wrinkle here is not checking if the project actually exists if the page already exists.

Test Plan:
- viewed a project wiki page
- viewed a prokect wiki page for a fake project and got a 404
- edited a project wiki page
- edited a project wiki page for a fake project and got a 404

Reviewers: epriestley…