Adding security.shared_users_group option for default users group

[costrouc]

Closes #957

This adds an option for security.shared_users_group which is by
default False. If True all users created after this option is set True
will be added to the users group. If any users were created before
this option they must manually be added to this group.

[danlester]

Code is fine I think, but not sure why the docs still refer to users and groups under security key in the YAML file (and also primary_group and secondary_groups). These have all been removed in favor of Keycloak managing them all in-cluster.

[danlester]

Also, we should add shared_users_group: true when upgrading to 0.4.0.
I have updated this plus the docs.

BTW you did not transfer the code that controlled shared_users_group by flags to qhub init, but I think that's fine... it was overly complicated for a user to worry about. We should impose a sensible default (which may be better as shared_users_group: false but that's for a UX discussion...).