Changes to the doc & django integration

[ewjoachim]

Relates to #934

• Remove the Django DB router. I'm not convinced it was a good idea, and it will complicate things.
• ProcrastinatePeriodicDefer should be readonly
• add PROCRASTINATE_READONLY_MODELS setting to ease testing
• Break Django doc into multiple docs
• Add a django test doc with unit & integration fixtures snippets (promoting InMemoryConnector)

Successful PR Checklist:

• [x] Tests
  • [ ] (not applicable?)
• [x] Documentation
  • [ ] (not applicable?)

PR label(s):

• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20breaking%20%F0%9F%92%A5
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20feature%20%E2%AD%90%EF%B8%8F
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20bugfix%20%F0%9F%95%B5%EF%B8%8F
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20miscellaneous%20%F0%9F%91%BE
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20dependencies%20%F0%9F%A4%96
• [x] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20documentation%20%F0%9F%93%9A

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
procrastinate/contrib/django
__init__.py
models.py
Project Total

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[ewjoachim]

the django reference doc is still failing