Support digital products

Some ideas:

• email with information is sent after publishing of digital product

• digital products support different states: registered, buyable, published

• digital_product as foreign key of item

• digital products provided through custom Nextcloud share link which is shown on digital products page (share has a limited time)

• digital products page accessible via login (logged-in order) or email link with crypto URL (see PDF generation)

• Show help text

• Show bought at

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