Coordinators' additional permissions over subgroups

If you're a coordinator, you now have the ability to join any closed subgroups, as well as make yourself the coordinator of that subgroup. (This doesn't extend to secret subgroups).

The help documentation has been updated here, in a new section on "Coordinating subgroups": https://help.loomio.org/en/user_manual/groups/membership/

(This change will be deployed later today)