MDL-47655 assign: Treat 'new' status as never submitted.

This fixes the recent activity callbacks to ignore submissions with a 'new' status.

The unit tests for recent activity were modified, because they were relying on the old behaviour of 'submitted' being the default status.

AMOS BEGIN
     CPY [submissionstatus_,mod_assign],[submissionstatus_new,mod_assign]
AMOS END