User groups

[nineinchnick]

Add an identity interface that would allow to read and assign groups. It would be a MANY_MANY relation. For reading available groups, a dataprovider should be used.

There could be a default group for new users.

It would be required to extend user managment UI. It would be similar to auth item assignment but with a different UI control because of potentially large number of groups (select2 multiselect).