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
Missing tests for SpecialistDocument#previous_editions
Refactor SpecialistDocumentRepository#publish! tests
Publication observers
SpecialistDocument support attributes method
Document rendering and export
Fix govuk_content_models integration issues
Extract header metadata and export
Bump govuk_content_models to 8.6.0
Fix up tests for CI

* Get asset manager URL from Plek (because this changes, obviously!)
* Ensure even local requests are disabled after JS tests
Rename SpecialistDocumentDatabaseExporter

to SpecialistDocumentExporter as it is more
generic than the name would suggest.
Re-instate single sign-on