Remove possibility for shared state in PublicTasks and friends

[agjohnson]

Similar to the changes in #3946, we should break out the task steps to a
separate class to encapsulate state. Tests first will help determine if this is
prone to the same problem as the project.task Task subclasses.

[stsewd]

I only see that we use PublicTask here

https://github.com/rtfd/readthedocs.org/blob/ba34164e28f56d0cb0ebc78cfb6ec801d0cae023/readthedocs/oauth/tasks.py#L24-L35

And we use that here

https://github.com/rtfd/readthedocs.org/blob/ba34164e28f56d0cb0ebc78cfb6ec801d0cae023/readthedocs/restapi/views/task_views.py#L58-L59

Not sure if we depend on this code in other parts (like on the .com site),
but as right now, I'd say that we are sharing states when doing this

https://github.com/rtfd/readthedocs.org/blob/ba34164e28f56d0cb0ebc78cfb6ec801d0cae023/readthedocs/core/utils/tasks/public.py#L89-L89

Also, I couldn't find any other based class tasks.

[agjohnson]

Yeah, so this probably needs to be broken out into a stateful object instead of inside the task.

As far as i can remember, we only use the public task during remote repository syncing. I think we had planned on using it for build triggering, so we could update the status on the build list/output pages, but never got there.

[stsewd]

So, we access the request object inside the class, we can't do something like #3946 (or at least I think so 😬). So, I thinking in using a bound task http://docs.celeryproject.org/en/latest/userguide/tasks.html#bound-tasks

[stsewd]

So, I was exploring this, I found that the same class (instance) is shared (same id), but the self.request object is different. I tested this creating two social applications and two users (one with GitHub and another with Gitlab) and using rdb. Maybe the request object isn't shared here? or I dit test this wrong?

Anyway, we can add this to #3973 and be 100% sure.