Fix logged-out Diffusion calls to Conduit

Summary:
Conduit doesn't currently have an analog to "shouldAllowPublic", so the recent policy checks added here caught legitimate Conduit calls when viewing Diffusion as a logged-out user.

Add `shouldAllowPublic()` and set it for all the Diffusion queries.

(More calls probably need this, but we can add it when we hit them.)

Test Plan: Looked at Diffusion as a logged-out user with public access enabled.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7380