messages in dashboard

*This implements part of #615*
*Note that this is rebased againt #677*

This allows to display messages online (in the dashboard). To allow that I had to remove some features.

-  It is no longer possible to select a sender name or email. It is taken from the message creator instead.
-  It is no longer possible to use placeholders in mass messages (no personalisation)
  -  I talk to someone from our project team and they said this was rarely used anyway

It is still possible to format email body and subject for each recipient individually. The message itself however is the same for everyone.

The number of recipients is displayed on the message view for everyone who can access that view (currently only sender and recipients). I am not sure if this leaks to much data.