Disable "Download Story as Plaintext" when disconnected

This is to help avoid confusion for people who want to backup their
current story after being disconnected from the server.

static/application.js

@@ -2367,7 +2367,9 @@ $(document).ready(function(){
 	});
 
 	button_downloadtxt.on("click", function(ev) {
-		downloadStory('plaintext');
+		if(connected) {
+			downloadStory('plaintext');
+		}
 	});
 	
 	button_load.on("click", function(ev) {