Private message integration: Create permission for each relationship type

In working on the "Send private message to all friends" issue, Berdir suggested that I create this issue as a follow-up. Basically, it would be great if there were a different permission on the permissions page for sending a private message to each relationship type.

For example, in many use cases, we may want to allow sending a user's "friends" but not to our default administrative relationship type. These per-relationship type permissions would be very handy.

--Ben