Massive overhaul regarding UX for adding content to a group

See this comment in particular.

Small feature/BC breaks that shouldn't affect that many people:

• Group Node (gnode) permission names have changed. An update hook took care of any roles using those permissions. You may need to manually adjust your views or code if you used any group permission to the likes of "OPERATION NODE_TYPE node" (e.g.: "view article node").
• We also had a bug where some 'update' permissions were incorrectly named 'edit', so we fixed that along with the above change. See this issue
• The configurable info text atop the membership / group node / etc. form has been removed. This configuration option has been made largely obsolete because the new flow allows you to skip the wizard when the second step is empty. If you still need such a block of text, we advise a simple hook_form_alter().
• The Group Node module has gone on a diet. Most classes have been removed in favor of the new, generic approach in Group. If you were extending any classes, please revise your code and/or open a support request.