Simplify (& fix) Message().__str__()

Simplify (& fix) Message().__str__()

We were still using len(self.get_replies()) for the __str__ summary of a
mail, but 1) len(Messages()) has just gone away 2) the number of replies
can not be retrieved when we got the message via search_messages()
anyway, and 3) it is likely quite expensive to pull all replies for all
messages that we display a summary of.

So we fix this by simplifying str(Message()) to omit the number of replies.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>