To make it more consistent with the rest of the Admin panel,
the CRUD for budget groups and headings has been changed
from the old "all-in-one" form to a separate form for each resource.
Changing a group’s `to_param` to return the slug instead of the id,
breaks many tests in the user facing interface
We should use slugs in upstream soon, but it should be done in a
separate PR, bringing the whole slug implementation from Madrid’s fork
and the corresponding test coverage
We were having a problem were some groups where not updating correctly.
That was because it was finding the first group with that name. However
we were looking for another group with the same name from another budget
Apart from the group_id, we also get the budget_id in the params for
updating a group. By finding groups within that budget we get the
expected behaviour
