Add the ability to import an existing database from InvoicePlane 1.
The user should either be able to migrate his database from within the setup or use the import tool after he set up the app for the first time. The setup variant should be preferred if possible to prevent duplicate work for the user.
It is essential that all data is migrated in some way. However, it may be possible that some data can't be migrated because some functions won't be implemented into IP 2.