Move to free app with in-app purchase

[BSVogler]

Offering, Completing, and Restoring In-App Purchases
Restoring for paid app: https://forums.developer.apple.com/thread/124587

I think the yearly, monthly and export view should be hidden behind some "premium" feature.

[BSVogler]

Things that need to be done in order to release

• Hide features behind a paywall
• allow buying and unlocking of the paywall
• allow restoring
• detect old users

[BSVogler]

Currently there is some code to detect old users, but I don't know how to test this, as the local build does not include a receipt.