Prevent database updates in input/form step

[Mark90]

It is illegal to save a subscription during an inputstep or form generator step, because these steps run "on the API side". The framework still allows you to do this.

A subscription can be saved:

• Explicitly by calling .save() on it
• Implicitly by making the step return it

We could investigate ways to prevent this in the inputstep and form generator.

Side note: create a decorator @form_step for the form generator functions?