SD-672 back-end support for user preferences

Added schema for user preferences regarding user mentions notification + behave test
thing is how we define what options there are. if it's server driven then there must be a way for apps to define the settings and server should send to client all the options that can be changed and it would make sense to group it on server. otherwise we make the client aware of the options it has and then it will also take care of grouping. so how should we do it?