Reimplemented dependency on nextcloud export feature

Due project license collisions on nextcloud-api-wrapper library used by nextcloud export pusher (WfExS-backend is Apache 2.0, nextcloud-api-wrapper is GPLv3+), that export pusher has been reimplemented around pyncclient-ext (MIT license), written by @jmfernandez to provide a subclass of pyncclient (MIT license) with the needed features.