Allow email to be retrieve by SSO ReverseProxy

If REMOTE_EMAIL header is set, use it as user email.
If REVERSE_PROXY_DEFAULT_DOMAIN is set but not REMOTE_EMAIL, use the current construct.

config.default.php

@@ -201,6 +201,9 @@ define('REVERSE_PROXY_USER_HEADER', 'REMOTE_USER');
 // Username of the admin, by default blank
 define('REVERSE_PROXY_DEFAULT_ADMIN', '');
 
+// Header name to use for the username
+define('REVERSE_PROXY_EMAIL_HEADER', 'REMOTE_EMAIL');
+
 // Default domain to use for setting the email address
 define('REVERSE_PROXY_DEFAULT_DOMAIN', '');