Bump govuk_content_models to 8.4.0

for new attachments functionality
SpecialistDocumentEdition#emergency_publish removed
Initial code and cucumber scenario for attachments

still need to add support for the markdown code
Process custom inline attachments tags
Put attachment helpers in module
Use Webmock during cucumber tests
Remove unecessary story
Bump content models to 8.4.1 to get fix
Fix preview with javascript and attachments
SpecialistDocument publishing refactor

* Move domain concerns from repository to domain object
* Use observer pattern to make pubishing actions more extensible