message: add map-provider GET param to Viewer app
diff: var collapseLeftPanel = getParam('collapse-left-panel', '[^&]+').trim() === '1';
  var pdbProvider = getParam('pdb-provider', '[^&]+').trim().toLowerCase();
  var emdbProvider = getParam('emdb-provider', '[^&]+').trim().toLowerCase();
  + var mapProvider = getParam('map-provider', '[^&]+').trim().toLowerCase();
  var viewer = new molstar.Viewer('app', {
  layoutShowControls: !hideControls,
  viewportShowExpand: false,
  collapseLeftPanel: collapseLeftPanel,
  pdbProvider: pdbProvider || 'pdbe',
  emdbProvider: emdbProvider || 'pdbe',
  + volumeStreamingServer: (mapProvider || 'pdbe') === 'rcsb'
  + ? 'https://maps.rcsb.org'
  + : 'https://www.ebi.ac.uk/pdbe/densities'
  });
  var snapshotId = getParam('snapshot-id', '[^&]+').trim();
  