generate logmessage for each alarm being sent

[fs1]

Each time an alarm or an er info ("Casus senden") was sent a log entry should be generated.
This log entry should inform the reader about: who initiated the message, who was the receiver, which type of message is it (tbd: is it better to log the whole message?) and at which time the message was sent.
This way we can handle complaints about the alarm-feature in a better way and we are also able to make statistics about the sent messages.

[robo-w]

I had a look into the data model of the log entries: It needs a database change for a clean implementation, so I suggest to postpone it for the next feature release. Currently the assigned units are logged in the PDF report, and also the "alarm sent at" message with initiating user and timestamp.

Summary: At the moment all relevant information is logged to the PDF report, but it could be put there in a better way. I think this could be combined with #68.