[core] Provide a way to safety valve Django settings

This is mostly just a thought:

There's often needs coming up to configure the Django framework that Hue sits atop, such as configuring cookie session lengths, ORM level properties, etc..

It may be worth having a general way via Hue's configuration layer (hue.ini, etc.) to inject arbitrary settings into Django's settings.py, such that folks can be unblocked in wanting to set such a change immediately than having to wait for Hue itself to add a wrapper config around the need (i.e. a code change).