Migrate to PositiveBigIntegerField and other minor fixes

1. Using PositiveBigIntegerField for user telegram ids
2. Add context: CallbackContext typing
3. Remove unused fields from User model
4. Add GetOrNoneManager. Example of using: user = User.objects.get_or_none(user_id=<some_id>)
5. Add AdminManager. You can select all admins via User.admins.all() instead of User.objects.filter(is_admin=True)