Make procrastinate.contrib.django.app a proxy

[ewjoachim]

Make procrastinate.contrib.django.app a proxy so that we use the same instance before and after it's ready

Relates to #934.

I don't think we need to talk about it in the doc. I'm not sure what kind of test would make sense.

Successful PR Checklist:

• [ ] Tests
  • [x] (not applicable?)
• [ ] Documentation
  • [x] (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
• [x] 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
• [ ] 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
apps.py
placeholder.py
Project Total

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