Lack of interest Allow callback base URL to be set in src/config.php

[Jake B.]

Going to resurrect this suggestion I made on the xf2demo board during developer previews. Basically, suggestion is to change:

public function getCallbackUrl()
{
   return \XF::app()->options()->boardUrl . '/payment_callback.php?_xfProvider=' . $this->providerId;
}

to something along the lines of:

public function getCallbackUrl()
{
    return (!empty(\XF::config('paymentCallbackBaseUrl')) ? \XF::config('paymentCallbackBaseUrl') : \XF::app()->options()->boardUrl) . '/payment_callback.php?_xfProvider=' . $this->providerId;
}

This will allow me to set:

$config['paymentCallbackBaseUrl'] = 'https://mysubdomain.ngrok.io/xf21';

without actually being redirected to my ngrok subdomain, getting logged out, and exceeding my request limit while testing payment processes