Revert "Efficiency improvements, support for view_thread in preview pane & not (ref #87 live repository)"

This reverts commit d577ae47d799fee3a11c3b762582b3eb97f8e284.
Fix view_thread for standard conversation thread view

Support for multiple DOM node classes to trigger an observer.

Allow both 'Bu' and 'Bs' nodes to trigger a view_thread event - Bu for
when preview mode labs addon is on, Bs for standard conversation view.