Invoicing

[EmCousin]

When a member subscribes to one or multiple courses for a given yeat, i.e creates a subscription in the database, that member will be sent an invoice "due" automatically in the subscription confirmation email, as an attachment, alongside the subscription form that is already attached.

The member should also be able to download their invoice at will from their dashboard, for each subscription available.

When an admin marks a subscription as paid (either by cash or check), the subscription should be regenerated so it is marked as "paid" and no longer "due"