Refactor email sending to reduce duplicated config.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
Only send "ready" signup response email for products that are ready.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
Change HTML doctype used by devpay_complete handlers to HTML5.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>