Change storage from local to sync

Change the default storage location from `local` to `sync`.
Some key points to consider:
- Setting up IDs for in [browser_specific_settings](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings), especially for Chrome and other Chromium based browsers
- Sync size(100KB) is lesser that local(5MB). Exception handler for _out of storage_ must be in place, warning the user. A fallback to local should be done.
- Research needs to be done to see how many tabs need to be saved before the storage fails, and if it is feasible.
- If the browser is not signed in or user wishes to opt out of sync, it should fall back to local
- Syncing when there is no connection
- Duplication of saves on `local` storage and `sync` storage **must** be avoided

Helpful links:
[storage.local](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/local)
[storage.sync](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/sync)