restore collections per-group content entity type

At some point between the last alpha we were on and the current beta, the UI for group content entities has changed.

My scenario is in addition to groups having user members and node content, there is a custom entity type 'resource' which is also group content.

I used to see a tab on a group called 'Resources', which listed the resources in the group, and provided a local task for users to create a new resource.

This is now replaced with a 'Related entities' tab, which lists all users/nodes/resources, and provides a local task which allows only adopting of existing entities.

For users, this is a massive step backwards in UX. Group members want to see a list of resources. Having nodes and users in there is surplus information. Furthermore, non-admins should not see a list of group members, so granting them access to this tab would be unsuitable anyway. And finally, there's no way to create a new entity in the group (though I admit this last bit may have been my own custom code that's no longer working...)