Previous commit removed notification models from the global collection
but did not actually update the existing notification.
This commit refactors the notification interface to allow us to update
it without re-surfacing the notifcation onscreen.
// FREEBIE
Only allow one notification at a time. Use a basic notification for
normal messages, and image notification for image messages, and a list
notification when there are multiple unread messages.
// FREEBIE
Add window.storage to the background page, which loads all data from the
'items' store in indexeddb, caching them in memory for synchronous
access, then override textsecure storage to use that in memory store.
