Limit federated sharing to trusted servers

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Is your feature request related to a problem? Please describe.
I'd like to ensure my users only share with servers I trust. Also, I like to limit federated sharing spam!

Describe the solution you'd like
Add an option to limit federated sharing to trusted servers

Describe alternatives you've considered
Right now trusted servers give some benefit (auto-accept and exchanged address books for auto-complete) but users can still share with non-trusted servers.