Fix help dialog changes to nav menu <li> entries

This PR partially addresses issue #230.

In a partial, &lt;style&gt; CSS content is applied to the whole page, not just the partial content.  In this case, an adjustment to &lt;li&gt; spacing for the help dialog was being applied to the exercise navigation list on the right side of the screen.

To solve this problem, class tags were added to the help-related elements and the CSS was changed to target them.