Sync edits with server

[yeban]

Current state of sync is documented here - 2a3550d.

Caveats include:

1. Persistence fails if user simultaneously starts working on another task (this can only be done by clicking on one of the submissions from user dashboard, so it's not a common use case for Afra).
2. Sync works only b/w two (or more) tabs of the same browser on the same system. In lines with modern web apps we will want user to resume in any browser on any system.

Resolving those will mean syncing localStorage with server.