[Bug 970046] Add reply-to support, and turn the firehose towards Kadir.

I refactored a bunch of tests, added some tests for reply-to, and then added reply-to support to our general email sending stuff, which made the tests pass.

Then in a separate commit (for easy reversion) I set `atopal@mozilla.org` to be the default reply-to address. :fire_engine:

To test: Trigger an email in the forums. I suggest asking a question, then aswering it as someone else. In the email that gets sent and/or printed to the debug console, look for a header like `Reply-To: atopal@mozilla.org`.

r?