Move long cache tests into correct file

The cache tests decorator is in utils.py, so test it there.
Change @token_required to @permission_required

As part of adding the admin app, we want to make sure that Stagecraft
is checking permissions with Sign-on-o-tron.

Whenever a client of Stagecraft makes a request that requires a bearer
token, Stagecraft now uses that token to check directly with Sign-on-o-tron.
Permissions derived from this check are used to allow or deny access to
Stagecraft resources.

- Change the dev-data-set-query-token to be a generic dev OAuth token
- Add a development user which is controlled using configuration
Remove the dataset permission

This is not needed. The data sets that are shown will be controlled by
the permissions of the backdrop_user.
Merge pull request #171 from alphagov/signon-permissions

Use Signon for permissions