Better relative urls support

[dscho]

Changes

• This improves support for relative URLs (which are used by default when running Hugo locally, although they are not used when deploying the site). This helps local testing.

Context

I was annoyed enough with these problems when testing #1888 in my development setup that I had to fix them, but since they have nothing to do with dark mode support, I've split them out into their own PR.

[To1ne]

I have no idea what this does, but sure !

[dscho]

I have no idea what this does,

@To1ne It fixes the $baseURLPrefix computation in application.js.

The idea of $baseURLPrefix is that the script will first determine the location of itself, know that it is in <base-URL>/js/application.min.js, and therefore now know the prefix to use when replacing, say, the download links based on the User-Agent.

The problem solved by this PR is that this computation did not work when the script location was ./js/application.js, as it expected something like https://git-scm.com/js/application.js or file:///home/To1ne/git-scm.com/public/js/application.js instead, i.e. a URL with a protocol.

The other problem solved by this PR is that when testing locally via script/serve-public.js and accidentally browsing to an invalid URL, it would only show the properly-styled 404 page on top-level URLs, but not in "subdirectories". For example, http://localhost:5000/does/not/exist would look like this:

Before	After

but sure !

👍