Scroll to top of window when switching sections or steps

In the builder, when the user changes workflow steps or story
sections, scroll the window to the top of the window.

Add ``window`` as a parameter wrapping the self-executing anonymous
function implementing the builder views.

I hoped that this would also work around the issue with drawer rendering
described in #904, but it doesn't and I'll need to address that
separately.

Addresses #910
Take scroll position into account for top margin

For the drawer and workflow step views, the ``margin-top`` CSS
property is set with JavaScript to accomodate the different
header heights between the "Build" and other steps.  This is
because of the header in the "Build" step also includes the section
list bar.

``AppView.pushDown()`` is called from ``AppView.render()`` in order
to push the subview and drawer container elements down below the
header.  To calculate the ``margin-top`` value, we need to take
into account the scroll position when calculating the offset of
the bottom of the header.

Addresses #904