Allow limited users communication with approved remote contacts

[mwild1]

Limited users are already restricted to communication with local users on the same Snikket instance.

In some cases an admin may want to allow communication with a list of approved contacts on other instances.

Relatedly, we may want to add a mechanism for admins to approve remote contact requests for limited users.

We should also consider whether the approved users should be approved for the whole instance, or on a per-user basis.