Provide a static splash page while NiFi is starting up

Many users do not realize that NiFi takes a few seconds to start up (depending on how many NARs are loaded). As of 1.6.0, on commodity hardware, NiFi takes about 16-20 seconds from $ ./bin/nifi.sh start to the UI being available to the browser (see output below).

I think it makes sense to immediately serve a static splash page on nifi.sh run that doesn't depend on any NARs being loaded, etc. and which shows a message that NiFi will be available in about N seconds. This can use a simple countdown from ~20, which seems to be consistent across builds, and use a JS timer to refresh when the API is available (AJAX call to a status endpoint). I've observed NiFi tutorials and new users get up and running and this is their first introduction to the app. Often they just get an error page and have to refresh and wait a few times in order to get to the canvas.

1.6.0, default build, unsecured, on 2015 MacBook Pro:

2018-05-10 16:40:44,734 INFO [main] org.eclipse.jetty.server.Server Started @33762ms
2018-05-10 16:40:44,747 INFO [main] org.apache.nifi.web.server.JettyServer NiFi has started. The UI is available at the following URLs:
2018-05-10 16:40:44,747 INFO [main] org.apache.nifi.web.server.JettyServer http://192.168.1.3:8080/nifi
2018-05-10 16:40:44,747 INFO [main] org.apache.nifi.web.server.JettyServer http://127.0.0.1:8080/nifi
2018-05-10 16:40:44,750 INFO [main] org.apache.nifi.BootstrapListener Successfully initiated communication with Bootstrap
2018-05-10 16:40:44,750 INFO [main] org.apache.nifi.NiFi Controller initialization took 16269179995 nanoseconds (16 seconds).