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
Update to changelog for #852 implementation
Merge branch '717-viewer_popup_captions' into develop
Update changelog for #717
Add favicons to builder and viewer templates

This is something that's been bugging me for a long time.

Factor favicon-related markup in base.html into favicons.html so
it can be included in templates that don't inherit from base.html.