rootUrlPathPrefix fix for cordova

Adds ROOT_URL for Cordova apps. Fixes 404 when loading images on mobile app.
no, first I remove leading `/` from `ROOT_URL_PATH_PREFIX`, then concatenate it with `ROOT_URL` and only then remove trailing `/` from concatenated string. So if there is no `ROOT_URL_PATH_PREFIX` it'll remove trailing `/` from `ROOT_URL`.