Add form object for specialist sectors

When creating a specialist sector tag we want to create a corresponding
artefact -- the artefact will let us register the route and guard
against route clashes.
Expose artefact slug errors in tag form

If there's an error with validating the artefact slug, eg because the
slug is taken or contains invalid characters, then we want to surface
that in the form object that builds the tag and artefact pair.
Update artefact name when tag is updated

When a specialist sector tag is updated we want to update the name of
its artefact to match the new title of the tag.
Bump govuk_content_models to 10.3.0

Brings in new artefact format `specialist_sector`.
Exclude panopticon-owned formats from rummager.

Currently only `specialist_sector`.
Exclude Panopticon-owned artefacts from the interface.

They have their own, specialised interfaces
e.g. /tags
Merge pull request #155 from alphagov/register-specialist-sectors-with-router

Register specialist sectors with router