Simplify custom policies before saving, and reject meaningless policies

Summary:
Ref T603. Do a little more sanity checking on custom policies, so policies like this:

[ Allow ] [ Users ] [ <no users> ]

...that don't specify anything and thus which aren't meaningful raise errors.

Test Plan: {F69570}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7314