Stop using fake non existing from email addresses

When a Skara bot wants to send an email on behalf of a github user that is currently not in the census, it has a fallback strategy of constructing a fake email address like this:

github.com+userid+username at openjdk.java.net

I'm not sure why this pattern was chosen, but these email addresses aren't valid and will soon cause problems for the email delivery method we are using for the Skara bots. The typical case for such emails is PR comments and an example can be found here:

https://mail.openjdk.java.net/pipermail/guide-dev/2020-May/000072.html

I propose we just get rid of these fake addresses and use a do-not-reply as the sender for such emails instead. We can still add the full name of the sender like we already do, so that humans can read it.