Remove `minutes to complete` and `uses government gateway` fields

https://www.agileplannerapp.com/boards/173808/cards/8894

* Migrate `minutes_to_complete` to `need_to_know` govspeak field
  * Use welsh when edition artefact is marked as welsh
* Completely remove government gateway data
* Remove inputs from transaction formats

See also:
https://github.com/alphagov/govuk_content_models/pull/275
https://github.com/alphagov/frontend/pull/707
https://github.com/alphagov/govuk_content_api/pull/208