Remove a broken test

This test should have been updated in an earlier commit (f9be046),
and would have failed if it had been. Since it tests behaviour
which the user edit form does not support, there is no need for
this test now.
Change user-organisation relationship to many-to-one

We initially chose to create a many-to-many relationship between
users and organisations because we thought that we would need that
flexibility when dealing with access controls for the Transition app.

After further discussions about Whitehall's existing use of user
membership of (single) organisations, and after thinking more about
what we need for Transition, we have decided that we have no
immediate use case for this being a many-to-many relationship and that
to do it that way at this stage introduces unnecessary complexity for
Whitehall and Transition.
Make the dropdowns on the user form consistent
Merge pull request #100 from alphagov/user_in_single_org

Change user-organisation relationship to many-to-one