Fix MathJaxView offset bug (exposed by rendering bug) scroll view wasn't displaying all contents.

[#74614804]

https://www.pivotaltracker.com/story/show/74614804
Add optional spinner between page transitions

This is invoked by setting window.showSpinner = true on
the index.html page.

This will be used in the Android version of the app, which
currently has very slow page transitions.
[#74052558]  Don't display grey bar when there are no image credits to display.

https://www.pivotaltracker.com/story/show/74052558
Merge branch 'master' into app-algebra