Option to restrict users to talk with other users of other groups

[tigermick]

Is your feature request related to a problem? Please describe.
It's not a problem but It will be cool to have an option which allow me to restrict users to talk with other users if they are not part of the same group.

Describe the solution you'd like
Talk could propose an option in talk settings to block users in terms of their group.
for exemple, I have 4 users : user1, user2, user3 and user4.
I have 3 groups : talk, family1 and family2.
talk to allow all users of this group to use talk.
family1 and family2 to distinguish 2 groups of users ; user1 and user2 in family1 and user3 and user4 in family2.
With this option, admin can prevent user1 and user2 to start a chat with user3 or user4 and conversely
user1 can just talk with user2 and conversely and user3 can just talk with user4 and conversely

[nickvergessen]

See #5442