set up channel search block for entry entities

There's useful code in ensureFacetBlock() which is about to get removed, including:

    // Figure out a region, because core still doesn't provide a way to get a
    // sidebar region.
    $regions = system_region_list($theme);
    if (isset($regions['sidebar_first'])) {
      $region = 'sidebar_first';
    }
    elseif ($matches = preg_grep('/^sidebar/', array_keys($regions))) {
      $region = reset($matches);
    }
    else {
      $region = system_default_region($theme);
    }

Hmm actually to preserve functionality, this might all have to be done in the Facets 3 issue!