Add enough padding to the bottom of the linear story viewer so that the last section can be scrolled to the top of the window.  Also implement the "sticky" header titles that show the currently viewed section in the header after the section's header scrolls out of view.
Add enough padding to the bottom of the linear story viewer so that the
last section can be scrolled to the top of the window.  Also implement
the "sticky" header titles that show the currently viewed section in the
header after the section's header scrolls out of view.

Closes #77

denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase