message to proposal

*This is rebased against #678 and #670 has been merged in here*
*This implements part of #164*

This implements sending messages to supporters, opponents and creators of a proposal. It is based on the new message API introduced in #678.

It tried nothing too fancy. I guess one could spend some time on DRY and on message topics (the message currently does not know that it was created from an proposal. It only knows its recipients). But I did not do any of that.