Add "Edit" and "Join" capabilities

Summary: These are general-purpose capabilities required by Projects.

Test Plan: Trivial.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3204

src/applications/policy/constants/PhabricatorPolicyCapability.php

@@ -19,5 +19,7 @@
 final class PhabricatorPolicyCapability extends PhabricatorPolicyConstants {
 
   const CAN_VIEW        = 'view';
+  const CAN_EDIT        = 'edit';
+  const CAN_JOIN        = 'join';
 
 }