messages in dashboard 2

*This is an updated version of #678*
*This implements part of #615*

This allows to display messages online (in the dashboard). To allow that I had to remove some features.

It differs from #678 in removing less of the old massmessage functionality.

A good start for reviewing this is probably `git diff feature-615-messages-in-dashboard feature-615-messages-in-dashboard-2`.