Cleanup Manager API

- `follow_feed` now has the same signature as `unfollow_feed`. It also works in the opposite way of the `unfollow_feed`. On one hand this change does not support backwards compatibility. On the other hand these methods are not mentioned anywhere in the documentation so I do not consider them as part of the public API. If this is the case then they could be even deleted as they are not used internally at all. What do you think about this?
- `unfollow_many_users` method is added.
- the documentation of `unfollow_user` and `follow_many_users` is updated.
should be: user_id stops following target_user_id