Enable background syncing for offline data

[alansemenov]

• If client is offline, all the changes to the list of to-do items must be stored in IndexedDB in the browser
• We should indicate that client is in offline mode
• Items, that are modified but not yet synced with the server, must be styled differently than the ones synced
• Register and tag the event listener (https://developers.google.com/web/updates/2015/12/background-sync) to trigger the background syncing once the client goes online