Postal address change and Authorized Signer Change for Identity

[zupzup]

1. Rework bill issue to new API
2. Rework fetch bills API (overview / home / detai)
3. Add new API for identity (including file upload)
4. Change postal address for identity/contacts/company to country/city/zip/address (also in bill chain etc.)
5. Change place of payment and place of signing etc. should be BillPlace { country, city }4. If an identity is created without all 4 postal address fields filled out, they are an "Authorized Signer", wo can only sign bills on behalf of a company, otherwise they are a "Bill issuer", who can do anything. IF the postal address is filled out later, the Authorized signer becomes a bill issuerThe postal address is not a mandatory field. We check country/city/address and based on that, you can either sign a bill yourself, or just with a company if you don't have one.The zip is completely optional.
   Create these as structs and use serde's flatten, so in the JSON, it's still a flat structure (https://serde.rs/attr-flatten.html)