Make Global Option for Friend-only Private Messaging

Right now, you can stop users from messaging you on a server by server basis only. While this is a good feature, due to the amount of servers people use, I think having a single point of entry under Account Settings for some options would help and be a lot neater.

Options would be something like: 

- Allow direct messages from friends only.

- Allow users from any shared server to direct message you.

- Only allow users from these servers below to direct message you.