Refactor code for readability

Make some code look a wee bit nicer.
Update site-wide HTML and CSS

The site-wide changes to HTML are all fairly minor, and involve the following
types of changes:
* Wrap the navigation links in an unordered list.
* Move the "Steve Jackson Games" banner into the <nav> tag.
* Greet the user by name.
* Link to the new CSS stylesheet.

The CSS is very modest. Only one stylesheet exists for the entire site.