Re-enable the sabredav browser plugin to fix sync client setup

Without the browser plugin it's impossible to setup the sync client (1.5.1)

From what I understand, to check the provided username and password the client makes a GET request to `example.com/owncloud/remote.php/webdav` with the browser plugin disabled sabredav will throw an error and the sync client thinks the credentials are invalid.

cc @DeepDiver1975 @dragotin @PVince81