Added target="_blank" so doesn't take you away

Otherwise, you can lose this page.

Author: dsblank

binderhub/templates/loading.html

@@ -43,7 +43,7 @@
 <div class="preview container">
 <p class="preview-text text-center">
 Here's a non-interactive preview on
-<a href="https://nbviewer.jupyter.org">nbviewer</a>
+<a target="_blank" href="https://nbviewer.jupyter.org">nbviewer</a>
 while we start a server for you.
 Your binder will open automatically when it is ready.
 </p>