Refined-github: Enable data sync for Mark as Unread feature

[shivapoudel]

Mark as unread data are not synced in other devices. Is this possible :)

[fregante]

This is possible. PR welcome for:

• changing browser.storage.local to browser.storage.sync
• adding a migration from local to sync if the user already marked some threads
• possibly handling storage limits because sync has only 4KB

Also #1032 probably has priority over this feature.

[vors]

Would it be nice if GitHub actually would support the setting read: false and not only read: true? :)

[sindresorhus]

Now that GitHub has implemented notifications bookmarking, we don't intend to add more to the "mark as unread" feature as it will probably become moot in the near future.