Fix for foreign key violation error when a department is deleted ...

after some users have added its department_id.

Solution based on blog "Set Foreign keys to null"
https://til.hashrocket.com/posts/jqilnp3d0h--set-foreign-keys-to-null

Fix for issue #2088

Add functionality for School/Department for an organisation. ...

Changes:
    - added a Department model whose instances belong to an org and require name.
    - added a DepartmentController and views for create, edit/update and a index view which is a tab
      of 'Organisational Details'.
    - added the optional attribute department_id to User model.
    - added Rspec tests.
    - updated app/controllers/registrations_controller.rb, app/models/org.rb and
      app/models/user.rb to get rid of outstanding Rubocop warnings

    Fix for issue #2088.