Edit ordergroup association in user admin (need feedback)

The aim of this change is to move the burden of ordergroup management from the user to foodsoft. When the ordergroup is not really relevant, I'd like it to blend in (without the user having to fully understand it).

Firstly:
* Allow creation of a new ordergroup with a new user (user admin, later also invitation form).
* Allow to change the current ordergroup of an existing user in the user edit form.

Secondly:
* Allow to edit the ordergroup fields of an existing user (address).
* Allow to create a new ordergroup for an existing user in the edit form.
* Allow to dissociate an existing user from an ordergroup.

I know several foodcoops that have a one-to-one relationship between users and ordergroups. Here the notion of the ordergroup could be fully hidden. In other cases, the ordergroup is either a single person or a collection of members; that admins need to separately create users and ordergroups is unnecessary work and complexity. This pull requests uses the user's name when auto-creating an ordergroup (and handles name conflicts in case of duplicate names).

What needs to be done:
- [ ] FEEDBACK needed, please (see comment below)
- [ ] verify ordergroup names are compatible with existing foodcoops
- [ ] cleanup commits