Allow groups to be used aliases for user mentions

As discussed here: http://meta.discourse.org/t/give-groups-full-names-and-allow-mentions/11168/3 : 

Allows admins to configure whether a group name can be used as an alias for users. If so (and depending on the access level), the user can then use the @-mention search feature for new topics and in private messages to select a group, which then gets replaced by the lists of members to mentioned directly.

The default is to be set that no one can use a group as an alias.