friend request privacy setting

Add a "who can send you a friend request" privacy setting.

This is needed to restrict people from getting unwanted friend requests. For example, if you have a dating theme, users may get many unwanted friend requests. 

The setting should allow users to restrict friend requests to "anyone, friends of friends, no one"

This can be used in conjunction with a "follow" system. Users will then be able to force users to follow them, rather than send an unwanted friend request.