SD-672 back-end support for user preferences

Added schema for user preferences regarding user mentions notification + behave test
Sure :)

On Wed, Sep 10, 2014 at 11:31 AM, Petr Jašek <notifications@github.com>
wrote:

> In features/preferences.feature:
>
> > @@ -0,0 +1,20 @@
> > +Feature: User preferences
>
> i would like to see the tests in order to see if the api is ok ;)
>
> —
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> <https://github.com/superdesk/superdesk-server/pull/239/files#r17348709>.
>