Migrate to mongodb with mongoengine as odm

Peewee with sqllite is nice and possible to get running really fast but unfortunately it doesn't support an easy way of doing migrations.
Since the data that is stored is minimal, largely non relational but with possible future schema changes coming up, it makes sense to switch to an document based database.

Mongoengine (https://github.com/mongoengine/mongoengine) seems like to most robust ODM in Python for mongodb.