Make "Mark as read" off state more persistent

continuation of commit 46b868a466

This update shall allow article "Mark as read" OFF to survive beyond the next "Mark these items as read", so you do not have to keep un-checking "Mark as read" on selected articles after every "Mark these items as read" or page reload.

Now when "Mark as read" is manually unchecked this state is recorded in browser cookies.

PS: not sure while file formatting (indentation) looks broken in github diff page.