Add whitelist for inviting users on your homeserver

**Description:**
Seeing the big waves of spam invites to CSAM and other illegal material to users on the element matrix rooms, I would like a way to whitelist only certain servers from sending invites to my homeserver. For example, I want to be able to get invites from the homeservers of friends, but not from other less reputable servers. I would still like to be able to _join_ public rooms such as the Synapse Admins room or Element Web rooms, but like to prevent getting _invited_ to rooms by people on external servers.