Alert user that projects/ is protected in Phriction

Summary: Fixes T3447 - adds check if slug is projects/ alerts user

Test Plan: Create new doc in Phriction. Type projects/whatever and get error

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3447

Differential Revision: https://secure.phabricator.com/D6901