Use navbar-nav class on text links only

[gabalafou]

This pull request fixes #1839 while also trying to simplify the way we use the Boostrap "navbar-nav" class.

Prior to this PR, we have both:

1. "navbar-nav" class applied to text links rendered by "navbar-nav" template
2. "navbar-nav" class applied to icon links rendered by "navbar-icon-links" template

This is what led to the bug in #1839 because I must have thought that the .navbar-nav selector was for the text links, not the icon links.

I see two possible ways of resolving this ambiguity:

1. Keep the navbar-nav class on the text links, but remove it from the icon links. This requires adding some style rules to the icon links so they look the same with and without the class. This is the option shown in this PR.
2. Keep the navbar-nav class on both sets of links, but change the name of the navbar-nav template to something else. This would require updating the docs, and I think it would also require theme adopters to update their theme config files, which is why I didn't choose this option.

[drammock]

by default I can't see any difference in the navbars between this PR and the current dev site. But if I hack in a change via the dev console to make one of the svg tags for a navbar icon have style=height:"2em" then I can indeed see that the icons are center-aligned on the PR build. Looking into the CI failures now.