Tune the websocket autoreconnect to minimise thundering herd effect

The client websocket code will automatically attempt to reconnect on
error or disconnection. This commit:

- Randomises the delay before reconnection
- Wait two seconds before attempting to reconnect (meaning the real
  delay will be between 2 and 4 seconds) -- this is an attempt to spread
  out reconnections over a longer period.
- Makes it stop retrying after 10 attempts (i.e. roughly 30 minutes)

h/static/scripts/websocket.js

@@ -34,7 +34,14 @@ function Socket(url) {
 
   function reconnect() {
     var didConnect = false;
-    var connectOperation = retry.operation();
+    var connectOperation = retry.operation({
+      // Wait 2s before attempting to reconnect
+      minTimeout: 2000,
+      // Don't retry forever
+      retries: 10,
+      // Randomize retry times to minimise the thundering herd effect
+      randomize: true
+    });
     connectOperation.attempt(function (currentAttempt) {
       socket = new WebSocket(url);
       socket.onopen = function (event) {