Remove AAIB and DFID importers and tests

These imports have been completed and so the code and tests are no longer
required.
Merge pull request #268 from alphagov/remove-aaib-dfid-importers

Remove AAIB and DFID importers and tests
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)