Refactor database schema

[kahlstrm]

This PR refactors the database schema as discussed and does other things.
The changes:

• Remove the parties table, have the addresses table, and store recipient information (name, email, and IBAN) on the invoices instead.
• Remove the due_date field from Invoice, as we do not want to implement it yet (it needs to be discussed with the treasurer, etc.)
• Adds sane defaults for fields such as invoice_status and creation_time.
• Cleans up some database fetching functions now that we don't need to join invoices and addresses for the list endpoint.

[lajp]

Thanks for the PR, as discussed this is a very welcome change and it's good to be done early on so that the required refactoring is only minor.

I left some comments I'd like you to address before merge