Add manuals

In order to unblock numerous agencies, we need to allow editors to create manuals for their content. This is the first step in that. 

In this PR we've added:
* `#index` for manuals
* `#new` and `#edit` pages (with `#create` and `#update` actions) for manuals
* validations for manuals

This relies on https://github.com/alphagov/govuk_content_models/pull/165 being merged.