SD-672 back-end support for user preferences

Added schema for user preferences regarding user mentions notification + behave test
server-side seems a better option to me, we cannot predict at this moment which preferences exactly we're gonna have, so far we are targeting e-mail notifications, but in the future there will be stuff related to, let's say: content, alerts, security, workspace, design, and more. Also, Superdesk's apps like liveblog may inject own preferences. Client should read from the server and render all categorized settings based on that.