beta: points timeline backend to local directory

There is no 'local-multiwrite' filebackend in beta. The timeline
extension has a fallback to {$wgUploadDirectory}/timeline" hardcoded in:

  // Get the backend to store plot data and pngs
  if ( $wgTimelineSettings->fileBackend != '' ) {
    $backend = FileBackendGroup::singleton()->get(
        $wgTimelineSettings->fileBackend );
  } else {
    $backend = new FSFileBackend( array(
          'name'           => 'timeline-backend',
          'lockManager'    => 'nullLockManager',
          'containerPaths' => array( 'timeline-render' =>
            "{$wgUploadDirectory}/timeline" ),
          'fileMode'       => 777
          ) );
  }

Setting $wgTimelineSettings->fileBackend to an empty string would
trigger the fallback and let us use timeline on the beta cluster.

Change-Id: I6cf66fe65b0dd50240bcb3a84fb7d01c3805037b