Reduce duplication in form controllers with disabled actions

[irisfaraway]

This came out of discussion here: https://github.com/DEFRA/waste-exemptions-engine/pull/241#discussion_r308131294

We have a lot of forms controllers that have disabled actions like this:

    # Overwrite create and go_back as you shouldn't be able to submit or go back
    def create; end

    def go_back; end

We should see if there is any way to reduce the duplication, potentially by creating a concern (eg CannotSubmit or CannotGoBack).

[cintamani]

This is all fixed now