Add a notice above the legacy section test helpers
De-sectionize the Content API artefact test helpers

Decouple the section tag from the Content API adapter's artefact test helpers.
Continue to stub the existing URL with 'tag' parameter

Some tests rely on the old behaviour still, and we shouldn't break it
if we can avoid it as it's still a valid request to make to the Content
API.
Only stub /with_tag.json + 'tag' param when section

As querying the /with_tag.json Content API endpoint using the 'tag' 
query string parameter is only valid behaviour for section tags,
make sure we only stub this when the section tag type is provided.
Merge pull request #111 from alphagov/content-api-moar-test-helpers

Split out the artefact test helpers for the Content API