Users settings (including filters and extensions) cloud sync

Multiple feature requests from different users. For example,
http://forum.adguard.com/showthread.php?7376

Protocol description:
https://gist.github.com/Mizzick/e954d1be3e2d51d84db314c00a6a7530

Relevant task in browser extension:
https://github.com/AdguardTeam/AdguardBrowserExtension/issues/315

**First step:**
sync through a file

**Second step:**
Sync through Dropbox/GDrive/iCould/etc

**Third step:**
Sync through AG account

**UI**
We should add "Settings" -> "Sync" section.

Checkbox:
[x] "Enable Sync"
Sync method: combobox with available sync methods.

Available methods:
File, Adguard account

If you choose File, we should let you choose file path and that's all.
If you choose Adguard account, we should let you either sign in or register a new account.

If sync is enabled, we should also show:
Last sync time: xxxxx
Sync now button