Fix XSS in custom fields management

Kacper Szurek (http://security.szurek.pl/) discovered an XSS
vulnerability in Custom fields management pages, caused by unescaped
output of 'return URL' GPC parameter. His report describes two ways to
exploit this issue:

1. using 'accesskey' inside hidden input field (see [1]) reflects XSS to
   the administrator in manage_custom_field_edit_page.php when the
   keyboard shortcut is actioned
2. using 'javascript:' URI scheme executes the code when the user clicks
   the [Proceed] link on manage_custom_field_update.php after updating
   a custom field

This commit fixes both attack vectors:

- properly escape the return URL prior to printing it on the hidden form
  field
- let html_operation_successful() sanitize the URL before displaying
  it, just like html_meta_redirect() does. In this case, if the
  string contains an URI scheme, it will be replaced by 'index.php'

[1] http://blog.portswigger.net/2015/11/xss-in-hidden-input-fields.html

Fixes #20956

This is a backport from master 3f2779b4c6dc8d465fb73c08cfa1d806184d2e79.

account_prefs_update.php

@@ -115,6 +115,6 @@
 	echo lang_get( 'operation_successful' );
 
 	echo '<br />';
-	print_bracket_link( $f_redirect_url, lang_get( 'proceed' ) );
+	print_bracket_link( string_sanitize_url( $f_redirect_url ), lang_get( 'proceed' ) );
 	echo '<br /></div>';
 	html_page_bottom();

manage_config_revert.php

@@ -65,7 +65,7 @@
 <div align="center">
 <?php
 	echo lang_get( 'operation_successful' ).'<br />';
-	print_bracket_link( $t_redirect_url, lang_get( 'proceed' ) );
+	print_bracket_link( string_sanitize_url( $t_redirect_url ), lang_get( 'proceed' ) );
 ?>
 </div>
 

manage_custom_field_delete.php

@@ -58,7 +58,7 @@
 <div align="center">
 <?php
 	echo lang_get( 'operation_successful' ) . '<br />';
-	print_bracket_link( $f_return, lang_get( 'proceed' ) );
+	print_bracket_link( string_sanitize_url( $f_return ), lang_get( 'proceed' ) );
 ?>
 </div>
 

manage_custom_field_update.php

@@ -64,7 +64,7 @@
 
 	echo lang_get( 'operation_successful' ) . '<br />';
 
-	print_bracket_link( $f_return, lang_get( 'proceed' ) );
+	print_bracket_link( string_sanitize_url( $f_return ), lang_get( 'proceed' ) );
 
 	echo '</div>';
 

print_all_bug_options_update.php

@@ -79,6 +79,6 @@
 	}
 
 	echo '<br />';
-	print_bracket_link( $f_redirect_url, lang_get( 'proceed' ) );
+	print_bracket_link( string_sanitize_url( $f_redirect_url ), lang_get( 'proceed' ) );
 	echo '<br /></div>';
 	html_page_bottom();

set_project.php

@@ -109,7 +109,7 @@
 <?php
 	echo lang_get( 'operation_successful' ).'<br />';
 
-	print_bracket_link( $t_redirect_url, lang_get( 'proceed' ) );
+	print_bracket_link( string_sanitize_url( $t_redirect_url ), lang_get( 'proceed' ) );
 ?>
 </div>