Add support for multilingual domain sites

Thanks for this patch! It works as expected. The only issue I had was that composer couldn't apply it against 8.x-3.7. I'm not sure if that is because of changes between 8.x-3.6 and 8.x-3.7 or because the patch header is a bit unusual. Regardless, attached is a reroll of this change that composer applies successfully.

Thanks for te patch
works as expected
Reroll for v4.0.3

Added a merge request that rerolls this for 4.x (4.19).

MR 56 needs work, the new url contains a double forward slash '/', one for the domain, one for the beginning of the path.

For me MR 56 works well, with the domain-based language negotiation. No extra slashes. But the language support does not work in this case anyway, e.g. having local at https://domain-en.local and corresponding prod's domain at https://www.domain.com there is no way to redirect from https://domain-fi.local to https://www.domain.fi

I also think that https://www.drupal.org/project/environment_indicator/issues/3092900 is a duplicate of this issue, since the problem reported there is fixed by this patch.

@yevko, please mark as reviewed and tested if you are sure about your testing.

@tess bakker we might need an issue about trimming the input to remove any slashes, and possibly a post_update.php implementation to update config for any sites who might have a trailing slash.

@tess bakker I've updated #3425491: Hostname field isn't a hostname but a base URL to include form validation that determines whether the user has added a trailing slash.
Additionally, I've added a database update in the merge request, that fixes any urls in configuration that happen to have a trailing slash. Please review, and mark this as RTBC and possibly that issue too if you can.

Fixed in 4.x to be released in 4.0.22 #3483897: [META] Release Plan for Environment Indicator Patch Release 4.0.22