Move policy config to "Policy" app and make policy.allow-public description scarier

Summary: Ref T603. We have a real policy app now, so put the config options there. Revise the description of the public policy switch to make it clear that enabling it immediately opens up the user directory and various other interfaces.

Test Plan: Viewed/edited config setting.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7154