Fix specialist tag validation.

Tags are only allowed to contain `/` if they are a
child tag.

This `govuk_content_models` version depends on a 
bump to `gds-api-adapters` which includes 3 major
version increases and thus three potentially
breaking changes.
1. Changes to the Content API adapter to decouple 
   tag methods from the `section` tag type.
2. Remove obsolete `curated_lists` method from
   Panopticon API. [unlikely to break anything]
3. Remove the ability to retrieve [business 
   support schemes] by identifiers.

This warrants careful testing before deploy.
Merge pull request #156 from alphagov/fix-specialist-tag-validation

Fix specialist tag validation.