Use LESS nested styles for spider story sidebar

This makes the code somewhat less verbose and more maintainable.

Addresses #873
Auto expand #story-nav element in viewer

Use ``display: inline-block;`` instead of ``float: left`` for
the children of the ``#story-nav`` element and remove the width.
This allows the element to grow automatically and accomodate
different numbers of buttons such as the "Topic Map" button in
the spider story viewer.

Addresses #873
Fix "Back to Connected Story List" button width

It shouldn't be restricted to 80px like the other buttons.
s/.summary-image/#summary-image/