MVJ-550 override_receivable_type in Rent (API)

[juho-kettunen-nc]

This implements the API part of override receivable type, where it is a property of Rent, instead of ContractRent like the other implementation in #771

[juho-kettunen-nc]

Still requires customer blessing to use this Rent version instead of the ContractRent, but this is the best version of the logic we have right now.

[henrinie-nc]

It looks good so I am approving, however two things could be improved if you agree:

• validate_override_receivable_type_value() could be refactored slightly to simplify it
• is_akv_or_kuva_service_unit_id() might be possible to replace by adding an attribute on ServiceUnit model for enablding and disabling override receivable type on rents.

[juho-kettunen-nc]

The CI build fails due to some migration 0082 that was present at some time in my feature branch, but has since been removed. I don't think there's even a commit for that anymore here. I wonder how to fix that in this pipeline, besides creating a new PR which would lose the comment history

EDIT: fixed by reverting formatting changes to service unit model help text

[henrinie-nc]

It (still) seems good, we could see if the validation logic can be simplified. The logic to validate seems quite complex, and if there is room for improvement it would make it easier to understand later on