Test scenario for long section scrolling in viewer

Addresses #852
Scroll to top of window when switching viewer sections

Update ``LinearViewerApp.showActiveSection()`` so it makes a
a call to ``window.scrollTo()`` to make sure that the user
always starts viewing a section at the top.

Add an acceptance test that explicitly checks viewing the story
in the popup viewer.

Addresses #852
Merge branch 'develop' into 852-viewer_scrolling
Merge branch 'develop' into 852-viewer_scrolling