Build-in `nikola serve` development server could support non-root sites better.

Python Version:

3.9.2 (not relevant)

Nikola Version:

Git commit b01c040298f.

Operating System:

Debian GNU/Linux 11 (bullseye) (not relevant)

Setup:

Configure a non-root SITE_URL, e.g., SITE_URL = "https://example.org/blog/".
Configure URL_TYPE = 'full_path'.
Have some content and build the site.
Start nikola serve --browser

Expected:

CSS and other supported files are loaded by the browser in the same way as they will be loaded after deployment to the production server.
It is possible to navigate links.

Actually seen:

As there is no CSS and other support stuff, the HTML looks raw and ugly in the browser.
Clicking links does not work.

Analysis:

The browser is sent to http://127.0.0.1:8000/ instead of http://127.0.0.1:8000/blog/.
The browser tries to load the support files from http://127.0.0.1:8000/blog/..., but the server tries to serve them from http://127.0.0.1:8000/.

Remark:

This bug report is for nikola serve what #3715 is for nikola auto.

getnikola / nikola