Added ownCloud backend for external storage

To make it possible to use the short ownCloud URL (without specifying
webdav.php or remote.php), a new backend is available for ownCloud.

The user must specify the host + context path in the "Url" field (which
is mapped to the "host" parameter) and the subdir to mount in the "Root"
field.

This is to prevent confusion because some users forget to append
webdav.php or remote.php to the WebDAV URL.

Fixes #4923
Merge pull request #5238 from owncloud/externalstorage-owncloudbackend

Added ownCloud backend for external storage