Give PhabricatorAuthPassword a formal CAN_EDIT policy

Summary:
Depends on D19585. Ref T13164. This is a precursor for D19586, which causes Editors to start doing more explicit CAN_EDIT checks.

Passwords have an Editor, but don't actually define a CAN_EDIT capability. Define one (you can edit a password if you can edit the object the password is associated with).

(Today, this object is always a User -- this table just unified VCS passwords and Account passwords so they can be handled more consistently.)

Test Plan:

• With D19586, ran unit tests and got a pass.
• Edited my own password.
• Tried to edit another user's password and wasn't permitted to.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19592