Allow custom request checker prior to displaying page

Summary: We want to allow a broader access to our installation but we need to check the request in that case.

Test Plan:
Created a simple PhabricatorRequestChecker returning a custom controller.
Verified that this controller is used when accessing any page.
Returned null from this checker and verified that all 209 Phabricator pages are accessible.

Reviewers: epriestley

Reviewed By: epriestley

CC: scottmac, aran, Korvin, btrahan

Differential Revision: https://secure.phabricator.com/D2488