mailer doesn't set the correct options while calling sendmail. if
``mailer.FROM`` is set to ``"Gitea Webservice" <user@example.com>``
sendmail is called like this
```
Sending with: sendmail [-F user@example.com -i alice@example.com]
```
and doesn't set the envelope-sender. the option ``-F string`` set the
sender full name and is used only with messages that has no ``From``
message header.
set the envelope sender address with ``-f sender`` (lowercase ``f``)
works for me (fedora 27, x86_64, postfix-3.2.4, go1.9.1, gitea-1.3.0)
ce4a52c22c