Draft: goabackend: split ownCloud/Nextcloud providers
Split the existing "owncloud" provider (actually Nextcloud) so that there are two branded version of the WebDAV provider.
Since existing accounts and consumers rely on the provider type name being "owncloud", the Nextcloud provider retains this for internal use while the ownCloud provider is "webdav-owncloud".
This is a re-based version of !11, that is derived from the
webdav
provider.
closes #12