Allow specifying groups of organizations to share opt outs

[ajohn25]

Is your feature request related to a problem? Please describe.
Opt out sharing is currently all or nothing. This does not allow great flexibility for sharing opt outs among a subset of Spoke organizations.

Describe the solution you'd like
Implement a concept of "opt out groups" which will allow sharing opt outs between organizations within an opt out group (and only organizations within the group)

Describe alternatives you've considered
Separate instances for each opt out group

Additional context
For our current needs, building a UI for this isn't totally needed, and can be deferred to later.