Add Django models

[ewjoachim]

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While I'm not sure it's a great idea to let people write things in the DB using Django, there's no harm in exposing our 2 tables as django models, so that people can easily add custom procrastinate monitoring to their Django projects.

So one model per table, and disable the save & delete methods (for now) as well as the manager update & delete. That should do the job, as a first approach.