Add-ons Redis Cache By Xon 2.18.0

• Restore XF2.1 support

• Fix high availability support where css updates would be sent to a possible readonly replica instead of the writable instance

• Disable behavior which caused css.php to return a 404 on an invalid template.
• For the old behaviour of returning a HTTP 404 response, set this config.php option:
• PHP:

  $config['svForce404OnEmptyCss'] = true;