genericize the user's friends blocks to be viewable from any node type/user page

We should genericize the user's friends blocks so that they are viewable from any given node type or /user page, not just usernode. I think we can accomplish this by removing the code that checks if usernode exists and putting in a configuration option that allows admins to choose which node types or user page to display the block on. Any further configuration can be done via the block module's visibility settings.