transport column in domain table

[dmorton]

In many hosting environments, it is necessary to store a transport address

to deliver mail to after being handled by Maia. One common method involves using a mysql transport maps with postfix; other MTAs may have similar methods.

Adding a column to maia_domains and a small interface to it would be very easy to add to Maia, and would not otherwise affect performance.

We could also add a sys config option on whether or not to present it on the domain screen.

Also, it could be combined with #304 to have Maia deliver straight to the transport; caution should be taken though, as rejections would mean the message gets scanned again before being delivered.