refactor(finance): modular rules

[chrisflav]

We replace the hardcoded financial rules by configurable rules: A bill generator is a rule that produces a list of bills based on the parameters of a statement. A automatic bill configuration is a lists of bill generators.

The rules are configured using JSONata.

Closes #251

Remaining TODOs:

• Complete the serialization of statements, excursions, members etc: Replace __all__ by curated list of fields, add fields for useful properties. Document the serialization interface.
• Add JSONata validation to BillGenerator admin (maybe allow testing directly in the admin change view?)
• Add translations, help texts, show models properly in admin.
• Improve visualisation of rule outputs in the overview.
• Clean up the old hardcoded logic and verify the new rule based setup can re-implement the old logic.
• Document the rule syntax and the available parameters, provide more examples.

[github-actions]

📚 Documentation deployed!

Documentation: https://chrisflav.github.io/kompass/cm-finance-refactor/

Docker Images:

• App: ghcr.io/chrisflav/kompass:pr-257
• Nginx: ghcr.io/chrisflav/kompass-nginx:pr-257

Add the awaiting-deployment label to deploy this PR to a staging server.