Formalize deprecation warnings
We use different ways for deprecation warnins ::ActiveSupport::Deprecation.warn, logger.warn etc. We should introduce one way that would allow us to say in which version the deprecation will be dropped. Also it should make it easier to find things to drop during release process. One note about API parameters, we also need to mark parameters or whole entry points as deprecated but they won't be removed (just not included in next API version). apipie-rails will introduce deprecated option for params shortly.