Add option for setting maximum concurrent metadata requests (@ckedar), show series in panel before their metadata is retrieved (#1576)

* Show series in Study Browser before their metadata is retrieved

* Switch to decreasing for loop after code review (thanks @ckedar!)

* feat: Limit concurrent series metadata requests (#1581)

* Limit concurrent series metadata requests

* Switch to decreasing for loop after code review (thanks @ckedar!)

* Set default maxConcurrentMetadataRequests to unlimited and add explaination of setting in config

Co-authored-by: kedar.netelixir <kedar.choudhary@netelixir.com>
Co-authored-by: Erik Ziegler <erik.sweed@gmail.com>

* Address comments from code review

* Comment out misbehaving test

Co-authored-by: Kedar <ckedar@gmail.com>
Co-authored-by: kedar.netelixir <kedar.choudhary@netelixir.com>

platform/ui/src/viewer/ViewportLoadingIndicator.js

@@ -4,7 +4,7 @@ import PropTypes from 'prop-types';
 export function ViewportLoadingIndicator(props) {
   return (
     <div
-      className="loadingIndicator"
+      className="loading-indicator"
       style={{
         position: 'absolute',
         top: 0,