Rebel can belong to multiple local groups

[mhulet]

Is your feature request related to a problem? Please describe.
Several members are active in more than one local group and want to be updated by both.

Describe the solution you'd like
That's fine if a rebel has to select ONE local group when registering. But once registered, coordinator should be able to define multiple local groups when editing rebels. This will update subscriptions in Mailtrain.

Describe alternatives you've considered
We may just add this rebel email to other lists on Mailtrain, but then data is not synced anymore, and the rebel is still only linked to one local group.

Additional context
None.