Correct request type for create edition

continuing with:
https://github.com/alphagov/govuk_content_models/pull/245

for a create edition action the request
type should be recorded as 'create'. this
ensures that we show the correct message
under 'History and Notes', and send the
correct notification out to editors when
changes happen.
Merge pull request #246 from alphagov/correct-request-type-for-create-edition

Correct request type for create edition