Celery: use an internal namespace to store build task's data

Use a `Task.data` (`readthedocs.projects.task.builds.TaskData` object) to store
all the data the task needs to work instead of storing it directly using
`self.`.

This is to allow us a simpler way to perform a clean _before_ (and/or _after_)
starting the execution of a new task and avoid potentially sharing state with a
previous task executed that may not be able to perform the cleanup.

The only thing we need to keep in mind is that when modifying these Celery
tasks, we _always_ have to add any new value inside the
`self.data.<my-new-attribute>` and not directly `self.<my-new-attribute>` to
avoid this problem. In the future, we could implement this protection at a code
level if we want to avoid this mistake.

See #8815 (comment)
See https://docs.celeryproject.org/en/master/userguide/tasks.html#instantiation

Author: humitos