Add strong params for case study

A lot of edition tests use the controller_attributes_for_instance method
to build a hash from the attributes of an edition which they send to the
controller method. This means a lot of attributes that we don't actually
want to be updateable are included, which raises an error because of the
setting of config.action_controller.action_on_unpermitted_parameters.

This commit stops that method being used in tests run for case study,
instead passing only the parameters of interest.
Add strong params to consultations

Expand the work of the previous commit to make consultations work.
Add strong params for detailed guides

Expand work from previous commits to cover detailed guides
Add strong params for news articles

Expand previous commits to cover news articles
Add strong params for policies

Expand previous commits to cover policies
Add strong params for speeches

Expand previous commits to cover speeches
Add strong params for national applicability

Expand previous commits to cover national applicability
Remove controller_attributes_for_instance

This is now unused