Allow anonymus access to Differential.

Summary:
Add possibility for not logged in users to browse and see Differential
revisions.

Test Plan:
Set 'differential.anonymous-access' config option to true, log out, you should
be able to browse Differential without logging back in.

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley, mareksapota

Differential Revision: 1044