Chore: Handle APP_URL with subpath correctly

[svenvg93]

Warning

If the user does not have the APP_URL set after this fix it will redirect them to http://localhost

📃 Description

This PR makes sure that we handle APP_URL with subpaths better.

closes #2596

Docs Update: alexjustesen/speedtest-tracker-docs#105

🪵 Changelog

✏️ Changed

• honor the APP_URL variable when generating urls inside the application

❔ Tests

This PR was tests with the added test cases.
As well manual with the following NGINX config

NGINX Config

server {
    listen 80;
    server_name localhost;

    # Main location for subpath
    location /speedtest/ {
        proxy_pass http://speedtest-tracker-laravel.test-1:80/;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_set_header X-Forwarded-Host $host;
        proxy_set_header X-Forwarded-Port $server_port;

        # WebSocket support
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";

        # Increase timeouts for long-running requests
        proxy_read_timeout 300;
        proxy_connect_timeout 300;
        proxy_send_timeout 300;
    }

    # Redirect root to subpath
    location = / {
        return 301 /speedtest/;
    }
}

Both the in App notifications and in all the requests the correct URL is used.

[alexjustesen]

Linking to #2541

[alexjustesen]

During testing this introduced a breaking change which we're not going to tackle at this moment.