Remove the imports of managers from models

[abhiabhi94]

Describe the bug

Currently, managers can be imported from models.

Steps To Reproduce

Something like this works:

from comment.models import CommentManager

Expected behavior

Managers should ideally not be available from the models. This was due to the namespace pollution caused by using * imports in __init__.py. #213 addresses the namespace pollution, but managers are still given there for backward compatibility. Remove these in v3.0.0.