email: send comment and pullrequest mails with the author's name in 'From'

When emails are sent for comments and pullrequest invitations, set the From
header to:
Author's Name (no-reply) <generic email address>

Using the name of the person that causes the email, makes the emails more
useful and interpretable for the recipient of the emails.
To avoid replies directly to the author, triggering an 'offline' email
discussion that is not visible in the Kallithea interface, a generic
'no-reply' email address is used instead of the author's email
address.
This approach is assumed to be accepted by spam filters, as several other
web services are using the same approach.

The sender used for other email types, e.g. password reset mails, is
untouched and remains the value configured in app_email_from.

The sender used for the SMTP envelope is untouched as well.

Based on code by Cedric De Herdt.