Split deliver_to Field for Forwarding

This used to be bug #5. I'm tired of bug #5. I'm closing bug #5. We can discuss this on the wiki, or on the mailing list, or whatever.

In the database, the aliases→deliver_to is stored as varchar(250) to store who the mail should be delivered to. This can be very non-optimal when the aliases table becomes BIG.

Today it can contain both user_id's and email addresses for forwarding.

I suggest that we change this parameter to an integer type. Then implement forwarding with a separate field using a deliver_to id of -1 to tell code to look for at the forward_to field.

This would increase efficency when searching for aliases with a specific id (ex: webmail) since it can be indexed as integer. It will also improve logic since forward_to is a separate field.