Provide a subscription model

[FlorianSW]

As a donator
I want to benefit regularly from the perks of a donation package without making a donation manually every month
So that I can focus on playing a game instead of remembering when I should to donate again.

Tasks:

• Add subscription possibility for user
• Make subscription configurable per package
• Send notification to user when subscription was renewed/payed
• Provide a link for a user to view an individual subscription (apart from on PayPal)
• Add a cancel and return URL to the subscription request to PayPal
• Validate incoming webhook events
• Prevent variable price packages to be subscribable
• Provide documentation (subscription configuration in packages, webhook requirement on aPayPal's side, ...)
• (Optional) Provide an overview of active subscriptions of a user (superseded by Add an overview of active and inactive subscriptions for donators #32)
• (Optional) prevent a user from having multiple subscriptions of the same package (superseded by Prevent a donator from having multiple subscriptions of the same package #33)
• (Optional) prevent a user from buying a package to which they are subscribed to (superseded by Prevent a donator from making a one-time donation to a package they are subscribed for #34)

[FlorianSW]

Will be included in the next "formal" release (which will be released depending on when some missing features are implemented).