Use relative URLs everywhere and remove <base> tag (#411)

There were a few places still using absolute URLs, including <link rel="icon">
tags, <link rel="alternate"> tags, and nav menu links. In addition, the
presence of a <base> tag caused the browser to absolute-ize all relative
URLs. This also removed "canonifyurls = true" from the example site config.

This is a problem for things like render.com, which will put the site on its
own domain for previews. For example, I have a preview site at
https://houseabsolute-com.onrender.com/. Ultimately this will be served at
https://houseabsolute.com/, but before I switched over I wanted to preview the
site.

The absolute URLs were being generated for https://houseabsolute.com/ because
of my baseURL setting in config and the presence of the <base> tag. This
completely breaks the preview site.