Introduce a repository registry

* Slug uniquness validation removed
* All document formats working
* Manuals in need of fixing
Manuals refactored to use repo registry

* Slug uniqueness enforcement completely removed
* Preview broken
Remove slug uniqueness validation code

Fix the broken preview issue where controllers where relying on a plain hash
of errors rather than a ActiveModel::Vaidations object.
Extract entity registries
Slug clash warnings for manuals and documents
Manuals can have multiple clashing sections

(if you hate yourself)
Dry up SpecialistDocumentEdition scoping
Proper error messages
Merge pull request #269 from alphagov/repository-registry

Extract more registries