Bs mapping validations

Add validations when creating a mapping or saving a document to ensure that mappings and fields on a document stay in sync.

There will need to be cleanup on Sieve before we can start using this version.
This should be implemented as a `before_save` validation in my opinion. As you want the validation to fail before the document is even persistent to mongo, which will not be the case in the present situation as `save_to_es!` is wrapped into and `after_save` mongoid callback.

Make sense?