- Convert `features` service to a class

 - Remove the unused `flagEnabled` method. Calls to this method have
   gradually been replaced by `store.isFeatureEnabled`.

 - Move the initialization of the background sidebar => annotator
   synchronization of feature flag information into an `init` method
   which is then called from the sidebar's entry point. This pattern is
   consistent with several other services. It is also necessary for services
   that are not depended upon by anything else because services are
   lazily instantiated.