Add support for multilingual domain sites

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.

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 🐛 Hostname field isn't a hostname but a base URL Active 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.

trackleft2 → changed the visibility of the branch 4.x to hidden.