Allowing for different, multiple email addresses

This patch allows for admins to set what email addresses map to which forums. That way, admins can define email address that are different than the standard "forum-forum@current-domain.com"; you can now have "something-else@anotherdomain.current-domain.com" and still have it map to a certain forum.

If admins have no need to set these, it should continue to work exactly as it does out of the box, with a default email address being set on install and on the creation of a new forum.