Remove final remnants of service registry

This lets us clean up any mention of services from the wiring, and
remove ManualDocumentAttachment and ManualDocument services into their
own registries, per others.
Merge pull request #216 from alphagov/remove-service-registry

Remove final remnants of service registry
Create class for manual builder
Manual repository factory refactor
Manual slug uniqueness validation

Breaks other manual validations
Fixed validations
Manual section slug uniqueness check
Add a <h2> for footnotes in manual documents

This allows manuals-frontend to correctly render an extra 'collapsible
section' for footnotes.
Merge pull request #217 from alphagov/add-heading-to-footnotes-for-manual-documents

Add a <h2> for footnotes in manual documents
Merge pull request #218 from alphagov/manual-slug-uniqueness

Manual slug uniqueness
Add missing require for development mode
Make sure version number is present

When we interact with PublicationLog we should make sure we alwayds have a version number amnd, for that matter, a slug as this will cause issues in the future.

This was being caught by Sidekiq when queueing publishing. It will also require migrations to be run in order to remove exisiting PublicationLogs again as there is some existing data we are aware of which currently breaks preview.
Merge pull request #220 from alphagov/bug/missing-attribute-on-publication-log

Make sure version number is present on PublicationLog
Merge pull request #219 from alphagov/add-missing-require

Add missing require for development mode
Add exporter for publishing manuals to Publishing API