Make the behavior of the lightbox more consistent with scroll

Makes sure that the advanced description is out of view when
the lightbox opens.
Remembers the scrollTop position that was set when the lightbox
opens, in order to restore it when it closes.

Change-Id: I534f7d718528d1e5a4911c68a524eb96cadeae90
Merge "Make the behavior of the lightbox more consistent with scroll"