Replace expectations with need_to_know

www.agileplannerapp.com/boards/173808/cards/8707

Expectations are replaced with a freeform govspeak field `need_to_know`.
Related: https://github.com/alphagov/govuk_content_models/pull/257, https://github.com/alphagov/govuk_content_models/pull/258, https://github.com/alphagov/publisher/pull/321

includes test fixes to accommodate a [breaking change in govuk_content_models](https://github.com/alphagov/govuk_content_models/blob/master/CHANGELOG.md#1800) where `specialist-document` artefact kind was removed.