Decouple Content API adapter tag methods from the section tag type

This pull request decouples the tag methods in the Content API adapter from the `section` tag type, so that they can be used to request tags of any type. Similar decoupling is also done to the test helper methods.

The existing methods for the adapter and test helpers are left in place so that current behaviour is maintained (although they have been refactored - their only purpose now is to call the new methods). 

This change may break tests in existing applications, as I have updated stub data used in the test helpers to return IDs including the tag type, as per the current behaviour of the Content API. Clients which still expect the legacy behaviour (where tags lived at `/tags/foo.json` instead of `/tags/section/foo.json`) will find their tests will break with these commits.