Allow editing of subject field on Group model in frontend

To do this, we have to call add_node_to_layout of the extensible form for editing groups.

added part::frontend prio::1 source::customer type::feature labels

assigned to @hansegucker

created merge request !215 (merged) to address this issue

mentioned in merge request !215 (merged)

closed with merge request !215 (merged)

mentioned in commit 3c0bc609