Send emails from the user, but include list headers so that reply-all works

Fixes #164.

According to a website on the public Internet, these headers will afford us this behavior:

If someone uses **Reply**, the email address of the user who sent the message will be in the _To_ field ("off list"). If someone uses **Reply All**, the email address of the user who sent the message will be in the _To_ field and the address in the _List-Post_ header will be CC'd (posted to Community).

We no longer set a _Reply-To_ field.
Only that it's more difficult right now. The batch mailer is very isolated and only has access to recipient variables and the mail object – no resources. I think that a refactoring of our mailer/notification stuff is in order to address this and some other issues, but I don't want to do it in this specific pull request.