Pre-render JSON silence payload when form is ready

When silence form is ready we should generate initial JSON preview, so when user clicks to show it there's something to show. Right now it's only generated if user changes something.

assets/static/ui.js

@@ -362,6 +362,7 @@ var UI = (function() {
                       event.preventDefault();
                   });
                   silenceFormCalculateDuration();
+                  silenceFormJSONRender();
                 }
             });