Move questions config things into config.py

from question_config.py and __init__.py.
[bug 936718] Implement infra for question statuses.

* Adds model manager to get questions with the following status:
** DONE
** RESPONDED
** NEEDS ATTENTION
** NEEDS INFO
** ESCALATED
* Adds helper to question model for setting and unsetting NEEDS_INFO.
[bug 939164] Fix product/topic edit form when one is missing.