Move navbars to separate template files

Now that the template engine is much more powerful, I think many style developers will start to use template inheritance more and more. The overall_header.html is of course one of the most important.

Would it be a good idea to move the <div class="navbar"> section to a new template file overall_header_navbar.html?

It's gotten pretty big.

Additionally, we could move the footer navbar as well (to overall_footer_navbar), for symmetry (as well as future expansions).