Add signed key rotation scheduler

Rotate signed prekey every 48hrs, waiting for online access if
necessary. After a rotation attempt is made, schedule the next run for
48hrs in the future.

We use a timeout to "wake up" and handle the rotation. This timeout gets
set on startup and whenever the next rotation time is changed. For
paranoia's sake, always clear the current timeout before setting the
next one.

Since new registrations necessarily upload new signed keys, we reset the
scheduled time to T+48hrs on `registration_done` events.

// FREEBIE

js/background.js

@@ -84,6 +84,8 @@
         if (open) {
             openInbox();
         }
+
+        RotateSignedPreKeyListener.init();
     });
 
     window.getSyncRequest = function() {