Allow "Default View" policies to be set to Public

Summary: Ref T603. Currently, we hard-code defense against setting policies to "Public" in several places, and special case only the CAN_VIEW policy. In fact, other policies (like Default View) should also be able to be set to public. Instead of hard-coding this, move it to the capability definitions.

Test Plan: Set default view policy in Maniphest to "Public", created a task, verified default policy.

Reviewers: btrahan, asherkin

Reviewed By: asherkin

CC: asherkin, aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7276