Opt-out of "mutual server" connection.

We're seeing a lot of great updates to help us combat spam and harassment within our servers. However, bots are still rampant in Direct Messages. A bot simply needs to join a large server and they can DM thousands of users en-masse with whatever spam or scams they want and we as server staff cannot do anything about it pre-emptively.

Simply give us an option to set our server to not count as a mutual server for the purpose of direct messaging a user. 

Users will still have the option to re-enable DMs from users within that server, but allowing us to disable it by default will save a lot of trouble.