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
* What: We need to generate slug on Budget, Group and Heading classes, validating its unique for its scope
* How: Adding a presence and unique validation using Budget always as scope.
