Fixes #317: Remove the opt-in handling for reply-by-email
Allow 'test' message to appear handled so we don't need to see it
Fixes #320, allowing use of html part if text part is not populated

Note that we will probably want to move to using the html part by default,
but it seems like too much risk right now. We can do that after a bit.
Added a watchdog for when we receive a test message