Make subscription API more RESTful
Right now we subscribe via post ":id/#{type}/:subscribable_id/subscription" and unsubscribe via delete ":id/#{type}/:subscribable_id/subscription" from a resource.
This is not very RESTful. I propose changing those endpoints to post ":id/#{type}/:subscribable_id/subscribe" and post ":id/#{type}/:subscribable_id/unsubscribe"