Site URLs in subdirs break various things

[rootwork]

I have checked all the prerequisites below and I'm yet experiencing a problem

• [x] Read the README.md
• [x] Have the extended version of Hugo installed
• [x] Used the exampleSite's config.toml as a reference
• [x] I have included my repository link. Instead of pasting long snippets here.

Describe the bug

I was trying to debug an issue where the link.svg images in hovered sub-heading permalinks were not showing up for me in situations where the site was in a subdirectory (e.g. a GitHub Pages site like username.github.io/projectname).

When I looked at how the path to that file is constructed, it's ${parentURL}${iconsPath}link.svg. I know iconsPath is fine because my "light switch" mode icons show up fine. So then I looked up parentURL:

// variables read from your hugo configuration
const parentURL = window.location.protocol + "//" + window.location.host + "/";

The comment implies that this is being read from Hugo config, but it's not -- it's constructed just looking at what's in the browser window, and it only reads the domain name, not subdir(s). Shouldn't this just use Hugo's own .Site.BaseURL value?

I can make a PR for this, but I wasn't sure how else parentURL might be getting used, or what assumptions there are about it.

[rootwork]

Assigning @onweru as I'm guessing this is your area.

[rootwork]

Update: This happens with rootURL used in the search feature as well, which is basically defined the same way (except no trailing slash). The search scripts can't access index.json, because it looks for it in <rootdomain>/index.json instead of <baseURL-including-subdir>/index.json.

I think both of these should be adapted to use .SiteBaseURL to allow for installs in subdirectories.

[rootwork]

I now have a PR for this in #324, so requesting @onweru's review there instead.