Bump govuk_content_models to add new kind

16.1.0 includes the new artefact kind for the
international_development_fund
Add International Development Funds schema
Add International Development Fund Model
Set up repository, builder and validator for IDF
Refactor doc params filtering to abstract doc

Since this is going to be used in other document controller types, it
makes sense to refactor this out to asbtract_documents_controller.
Views, controllers and services for IDFs

This commit has a lot of stuff in it, but it's only adding controllers
and views for International Development Funds along with their
associated service registries (and for attachments too) along with a
view adapter.
Add permission check for DFID editors
Adding observers for IDFs
Move User cucumber step to access_control_steps
Testing International Development Fund workflow
Merge pull request #184 from alphagov/add-international-development-funds

Add International Development Funds