Rebuild backend to use postgresql

[jaylett]

Redis was an interesting experiment. But I think we're agreed that it doesn't give us anything other than a lot more code emulating a rubbish ORM, compared to just using the Django ORM and postgresql.

[georgebrock]

I played around with this a bit this afternoon, starting to move the Mission model over from Redis to Postgres. I quickly realised that the copy attribute would be a good candidate for a PostgreSQL JSONB field, especially given all the little optional bits and bobs we've added there over the years to support the quirks of this or that mission.

Unfortunately, we're on Django 1.8 and django.contrib.postgres.fields.JSONField wasn't added until 1.9, so I started upgrading in #255.