Change MultiRecordQuestion to MultiRecordCategory.

- Makes more sense modelling it this way.
- This will break the existing backbone logic for Multi-Record.
- Introduce STI for categories.
Remove MultiRecordQuestion specs.
Let the factory create CategoryViews.

- The category has type `null` now.
A category's `type` is mass-assignable.
Remove the backbone MultiRecordQuestion views and model.
Add guard clauses to the Question Factory's methods to make an `undefined` type `null`.
Category#as_json includes the type as well.