Skip group type selection page when creating a group for users that who can only add one group type

Motivation

With the introduction of the flexible group type it makes sense to disable the creation of other group types for certain user roles.
When disabling all but one group type, it's not user friendly to be sent to a page where there is just one option.

Solution

When a user can only add one type of group, we adjust the route of the menu item "New group" from group add page to the group add group type page.