Remove mins to complete and government gateway

The fields aren’t being used and can be accommodated by the recent
“need to know” field.
Remove min/gateway fields from models and data

* Bump content models to bring in new edition models
https://github.com/alphagov/govuk_content_models/pull/273
* Add migration to unset government gateway fields
* Add migration to move minutes to complete to need to know field (put
at start of list as frontend currently does)
Use welsh minutes message where appropriate

Tweak migration to use welsh when the edition is welsh
Merge pull request #338 from alphagov/remove-surplus-transaction-fields

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