Switch mail sender address and update templates

[flodolo]

We need to start using no-reply@pontoon.mozilla.com to send emails.

I assume we should also add a blurb to not reply to this address, and if they want to get in touch they should reach out to pontoon-team?

[mathjazz]

Do we want to use the no-reply address in the From field or just Reply-to?

[flodolo]

I don't have a strong preference. Maybe reply-to is enough?