Django rewrite: server-side sessions & configuration

[clpetersonucf]

Materia leverages server sessions in FuelPHP to store certain elements of user data between contexts: these include authentication state, and even more critically, LTI play session data.

Django has native server session support with a similar range of session engines:

1. The database (this is currently what Materia uses, to allow multiple application instances)
2. Local filesystem
3. Server cache

In the FuelPHP version, the selection of server session driver is configurable, and ideally this should remain the case in django.