This refactors default_current_user_provider in a few ways:
- Introduce a generic `api_parameter_allowed?` method which checks for whitelisted routes/formats
- Only read the api_key parameter on allowed routes. It is now completely ignored on other routes (previously it would raise a 403)
- Start reading user_api_key parameter on allowed routes
- Refactor tests as end-end integration tests
A plugin API for PARAMETER_API_PATTERNS will be added soon
