R1:5899ae08b32b

Add storage for custom policies

Summary: Ref T603. Allows custom policies to be saved. No integration with policy controls yet.

Test Plan:
mysql> select * from policy where id = 3\G
*************************** 1. row ***************************
id: 3
phid: PHID-PLCY-e4v2fnbyuibi4supl5tn
rules: [{"action":"allow","rule":"PhabricatorPolicyRuleAdministrators","value":null},{"action":"allow","rule":"PhabricatorPolicyRuleProjects","value":["PHID-PROJ…