Added a SAML_BEHIND_PROXY env, which must be set if Part-DB is behind a reverse proxy when using SAML

The php-saml library use its own logic to determine the current path, and we need to set the use_proxy_vars so that  it respects the proxy FORWARDED headers.

This should fix issue #436

config/packages/nbgrp_onelogin_saml.yaml

@@ -5,6 +5,7 @@ parameters:
   saml.sp.privateKey: '%env(string:SAML_SP_PRIVATE_KEY)%'
 
 nbgrp_onelogin_saml:
+  use_proxy_vars: '%env(bool:SAML_BEHIND_PROXY)%'
   onelogin_settings:
     default:
       # Basic settings