Add artefact kind for manuals

[tommyp]

This adds the new Manual artefact which alphagov/specialist-publisher#76 depends on.

[bestie]

I think we should consider adding a couple of methods to ManualEdition as suggested in my comments on alphagov/specialist-publisher#76

[bradwright]

This field is specific to HMRC's tax manuals (I don't see the service manual using this, for instance). We should consider the object hierarchy appropriately.

[evilstreak]

This field is to tie the editions for one manual together. It's generated by the specialist-publisher before a new manual is created and then used for each subsequent edition.

We do the same for SpecialistDocumentEdition: https://github.com/alphagov/govuk_content_models/blob/master/app/models/specialist_document_edition.rb#L8

[bradwright]

OIC. Thanks @evilstreak

[bestie]

Closing due to removal of ManualEdition collection from content models. This collection will live in Specialist Publisher.

We will later add another collection of rendered manuals for use by the Content API.