Cloud-based profile synchronization

[Dither]

Ok, now that I've tried using browser.sync it became obvious that ~100 KiB is not nearly enough to store anything except preferences and minimized subscription lists. Full message database can be several MiB in size and can't be synchronized this way.

So maximum that can be achieved is to store minimal version of each entry {entryID, unread, visited, deleted, trashed, pinned} separately and combine data from both sources on demand which will annoyingly complicate things as there are 2 entirely different data storage used browser.sync and indexedDB and single BackBone model for Item.

In the perspective, better idea would be some kind of button/timeout for syncing instead of doing it automatically because of possible data overwrite/corruption.

I will hold off the idea for now until there will be some good ideas on this.