Add means to specify address type when sending messages that aren't replies.

In #440 a lot of the delivery_class things got cleaned up but we now need a way for the user to specify the address type to use when sending messages to contacts. Options are:

* Specifying the address type in the message sending form (I think this is my preferred method currently because it allows the user to specify the address type each time).
* Including the address type in the conversation config (this requires the user to know to update the config before sending messages to a new type of address so I'm less keen on this).