group_controller() on IGroupForm not in interface

[metaodi]

In plugins.py, there is a call to a function group_controller on IGroupForm plugins.

This method is not mentioned on the IGroupForm interface. This leads to a strange bug: I'm using ckanext-scheming and this implements IGroupForm for a custom organization schema. But because this method is not present, it will use the group controller instead of the organization controller to generate links (e.g. for the pagination).

I guess this method just went missing and should be added to the interface and can then be implemented by extensions as needed.

[davidread]

Sounds good, are you ok to implement this?

[metaodi]

Sure, just wanted to ask first if I understood the concept correctly. PR follows.