Fix for missing scheme on ADMIN_APP_URL

[antonioribeiro]

Description

If we set the application URLs as

APP_URL=https://domain.com
ADMIN_APP_URL=admin.domain.com

Twill Exception handler will end abnormally the application if an exception occurs:

[2021-09-17 18:15:42] dev.ERROR: Uncaught ErrorException: Undefined array key "host" in /sites/domain.com/vendor/area17/twill/src/Exceptions/Handler.php:40
Stack trace:
#0 /sites/domain.com/vendor/area17/twill/src/Exceptions/Handler.php(40): Illuminate\Foundation\Bootstrap\HandleExceptions->handleError()
#1 /sites/domain.com/vendor/laravel/framework/src/Illuminate/Foundation/Exceptions/Handler.php(569): A17\Twill\Exceptions\Handler->getHttpExceptionView()
#2 /sites/domain.com/vendor/laravel/framework/src/Illuminate/Foundation/Exceptions/Handler.php(478): Illuminate\Foundation\Exceptions\Handler->renderHttpException()
#3 /sites/domain.com/vendor/laravel/framework/src/Illuminate/Foundation/Exceptions/Handler.php(356): Illuminate\Foundation\Exceptions\Handler->prepareResponse()
#4 /sites/domain.com/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(119): Illuminate\Foundation\Exceptions\Handler->render()
#5 /sites/domain.com/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(96): Illuminate\Foundation\Bootstrap\HandleExceptions->renderHttpResponse()
#6 [internal function]: Illuminate\Foundation\Bootstrap\HandleExceptions->handleException()
#7 {main}
  thrown {"exception":"[object] (Symfony\\Component\\ErrorHandler\\Error\\FatalError(code: 0): Uncaught ErrorException: Undefined array key \"host\" in /sites/domain.com/vendor/area17/twill

For this to work, people need to remember to add the scheme to the Twill's admin URL too:

APP_URL=https://domain.com
ADMIN_APP_URL=https://admin.domain.com

This is happening because parse_url() needs the scheme to correctly parse the URL, so this is good as it parses and finds the host:

parse_url('https://admin.domain.com');

[
     "scheme" => "https",
     "host" => "admin.domain.com",
]

But without the scheme we get:

parse_url('admin.domain.com');

[
     "path" => "admin.domain.com",
]

This PR a quick fix for it, but this problem is actually deeper and needs us to fix it for good on 3.0.

[pboivin]

Thanks @antonioribeiro! Closing in favor of #1213 which removes the parse_url() call