[bug 936718] Implement infra for question statuses.

The first commit moves config-y things for the questions app into config.py for consistency with what we have been doing recently.

The second commit is the actual bug work:
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.

There are tests but this stuff isn't used for real until we do the UI changes.

r?