#663 Unread/read flag is global

[Geir Bćkholt]

the flag indicating whether a ticket is read or unread is global to al users.
This makes it impossible to use the flag for knowing if you have read mail
or not, as someone else might just have browsed by it and inadvertedly
marked it as read.

two levels of solutions exist.

1) mark mails as unread when assigned
2) Make the read-flag linked to the current user with a separate table in
the DB.

1 should be implemented anyway as it will take care of messages
bouncing back and forth between users being marked correcly