fix justification of nav links and improve performance

[cconcannon]

The layouts/_default/single.html page has links at the bottom to navigate to/from additional posts. These <a> elements were given a class that was not specified with any styling in the styles.html file. As a result, the links would stack on top of each other in a left-justified manner like so:

I modified the page to use an existing justification attribute for each link, and I added constraints for those attributes (via the width property) so that the links will display side-by-side. Here's a screenshot of the result:

[cconcannon]

I added additional changes to the header partial so that the rendered html always uses the post metadata description and so that the font CSS loads in a non-blocking manner. These improvements bump up the default Lighthouse performance scores