Add offline test mode / config flag

This pull implements
1. A offline flag as described here [T76885](https://phabricator.wikimedia.org/T76885)
2. It also moves the config loading code to rbUtil.js


This function doesn't directly call out to the web, but creates a request that gets handled by [web.js](https://github.com/wikimedia/restbase/blob/master/lib/filters/global/web.js).  This `if (offline)` block might fit better in web.js.