Improvements of addon license model

[aerni]

I've been missing some flexibility around the license model of addons on the marketplace. For smaller addons, the current model works great. But for bigger addons with ongoing development, the current model feels too limiting.

Currently, we can make a license work until a specified version. But this is not always a great option for existing customers. I don't essentially want to charge the full price again for an update.

I wish there was an option for the following license models:

• Initial license fee with one year of updates and a yearly license fee for updates thereafter.
• License that works until a specified version. Option to have a discounted price for updates for existing customers.

[wm-simon]

But please add subscription option for the yearly license fee. It's already annoying to buy the statamic license for every website every year.