cmd/strelaypoolsrv: Hardcode a usable maps API key (fixes #5296)

Yeah it's not the most beautiful solution but it works for now.

cmd/strelaypoolsrv/gui/index.html

@@ -187,7 +187,7 @@
     <script type="text/javascript" src="//code.jquery.com/jquery-2.1.4.min.js"></script>
     <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.8/angular.min.js"></script>
     <script type="text/javascript" src="//maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
-    <script type="text/javascript" src="//maps.googleapis.com/maps/api/js"></script>
+    <script type="text/javascript" src="//maps.googleapis.com/maps/api/js?key=AIzaSyDk5WJ8s7ueLKb99X5DbQ-vkWtPDAKqYs0"></script>
   </body>
 
   <script>