Block mailmanager.cityweb.de reports

It seems we are getting enough reports from mailmanager.cityweb.de to consider this spam now:
https://github.com/webcompat/web-bugs/issues?utf8=%E2%9C%93&q=is%3Aissue%20mailmanager.cityweb.de

We should update the [spam list](https://github.com/webcompat/webcompat.com/blob/master/webcompat/views.py#L168) to include this domain and change the message to be more generic.