Add code to User to get their organisation
Add User.gds_transition_manager?
Add User.can_edit?(organisation)
Display a link to "my" organisation in the topnav
Indicate that you can edit mappings for an org
Change gds_transition_manager to admin

"gds_transition_manager" didn't describe everyone in the group. Crudely it will
probably be anyone at GDS. Potentially, we could set everyone at GDS to be a
member of the GDS organisation in Signon, and then have GDS be a special case
in Transition, but because Whitehall doesn't have GDS as an organisation, we
can't really do this.
Organisation-based access control for mappings

Prevent editing of an organisation's mappings unless:
  * the user is an Transition app "admin" according to Signon
	(eg GDS Transition Managers, GDS content editors)
  * the user is a member of the organisation
  * the user is a member of the parent organisation
Add a cucumber test for access control
Add more complete controller tests