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-cwovm5gn2ilubjehcdgd"]},{"action":"allow","rule":"PhabricatorPolicyRuleLunarPhase","value":"new"}]
  defaultAction: deny
    dateCreated: 1381437466
   dateModified: 1381437466
  1 row in set (0.00 sec)

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7282