Phriction - make the check for project sub pages more fine-grained

Summary:
we were just checking if projects/ was in the URI before barfing. Use some more fun utility functions such that we only complain if there is no project.

Fixes T4071.

Test Plan: made a subpage under a project - success! tried to make a project wiki page where there was no project - successful failure! tried to make a project wiki sub page where there was no project - successful failure!

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4071

Differential Revision: https://secure.phabricator.com/D7527