Add the register to vote done page promotion toggles
Add the register to vote done page promotion toggles

- We're adding a new promotion to transaction done pages to encourage
  people to register to vote. This requires some methods adding here so
  that it all works across publisher and frontend.
- We had to add `self.class.default_presentation_toggles['...` to the
  two key methods because we changed the default values, but the old
  organ donation promo pages wouldn't know about the new register to
  vote keys, so if one is nil, use the new defaults.

alphagov/govuk_content_models