Merge lp://staging/~edwin-grubbs/launchpad/bug-568390-cyclic-membership-error into lp://staging/launchpad

Summary
-------

The TeamReassignmentView changes the owner of a team and then adds the
new owner as a member of that team. The view now disipays an error if
this creates a cyclical team membership, so that the model won't raise
an exception.

Implementation details
----------------------

The ObjectReassignmentView.isValidOwner() was not very flexible, since
it shouldn't be used to set a custom error message, so I changed it to
validateOwner(). This gets called at the end of
ObjectReassignmentView.validate(), after a new team has been created, if
the user is not using an existing team. isValidOwner() does not appear
to be used in any of the other subclasses of ObjectReassignmentView.

    lib/lp/registry/browser/objectreassignment.py
    lib/lp/registry/browser/person.py
    lib/lp/registry/browser/tests/team-reassignment-view.txt

Tests
-----

./bin/test -vv -t team-reassignment-view.txt

Demo and Q/A
------------

* Open https://launchpad.dev/people/+newteam
  * Create three new teams.
  * Add teams as members, so A is a member of B is a member of C.
  * Open https://launchpad.dev/~A/+reassign
    * Try to make C the owner of A.
    * It should display an error that this will cause a cyclical team
      membership.