Improve mobile cosmetics around navbar
This MR includes various cosmetic changes to the frontend:
- fixes a bug introduced by !80 (merged) where the "place-holder" element under the sticky desktop navbar was visible on mobile.
- hides the "Edit knowledgebase" buttons on categories on mobile since editing GitHub content on mobile is janky at best anyway
- change mobile nav to be white as well, following !80 (merged) changing the desktop nav to be white.
- word-wrap long category names in Breadcrumb elements across all pages
- fix the website logo being a
<a>
element and therefore causing a hard page reload; we now use<Link>