Bug: Log navigation bar item disabled for historical log display

When displaying a log for other than the current month, the ``Log'' item in the navigation bar was highlighted and disabled, which prevented it from being clicked to display the current month. I added logic to test whether the current month is displayed (this, as the handling of the ``{\tt now}'' month specification, is presently based on UTC and needs to be reviewed with regard to {\tt tzoffset}). If the month displayed is not the current month, the ``Log'' item may now be clicked to jump to the current month. (Reported by Rob Campbell.)