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

[aknrdureegaesr]

Environment

Python Version:

3.9.2 (not relevant)

Nikola Version:

Git commit b01c040.

Operating System:

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

Description:

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.