Allow trusted proxy via APP_TRUSTED_PROXIES (#9196)

* Allow trusted proxy via APP_TRUSTED_PROXIES
Set to '*' by default to emulate legacy behavior.
Set up doc describing environment variables

* Create helper to parse environment variables into arrays properly.

* Update doc blocks

app/Http/Kernel.php

@@ -19,6 +19,7 @@ class Kernel extends HttpKernel
         \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
         \App\Http\Middleware\TrimStrings::class,
         \Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,
+        \Fideloper\Proxy\TrustProxies::class,
     ];
 
     /**