make pay/close dialog have a "message" field

make the dialog have the subject/message stuff from the "message" dialog (in addition to the feedback textbox)
[ Done ]

make the message subject default to "thanks!", and text default to "thanks for your help, I am now closing the contract, and paying you!"
[ Done ]

when "pay/close" is clicked, make a rpc call to send a message with the message subject and text (in addition to what it already does)
[ Done ]