Fix javascript variable type (#280)

Fix the case when the value is missing and is breaking the page script entirely.

priv/index.html.eex

@@ -7,7 +7,7 @@
         var xhttp = new XMLHttpRequest();
         xhttp.onreadystatechange = function() {
           if (this.readyState == 4) {
-            window.location.href = <%= base_path %>
+            window.location.href = "<%= base_path %>"
           }
         };
       xhttp.open("DELETE", link, true);