Deepen the feature-set for subscriptions / e commerce

[Martinsos]

Right now OpenSaas brings some basic subscription management + Strype payments.
Besides that, there is a lot of other related logic that most of the SaaSes need, that OpenSaas could potentially offer, like refunds, downgrading, ... .

Here are some suggestions from @cassofthenight :

If a customer has just successfully completed a payment, they should not be presented with buy option for the same subscription again as this does not make sense in most standard scenarios.

Furthermore, for example, an upgrade should cause the customer to pay a gap pro-rata rather than have to pay the full amount again, and a downgrade if allowed should either 1) refund the customer the gap pro-rata or 2) not refund anything.

Also, there should also be an option in terms of any subscription changes being deferred to the next payment cycle or being implemented on a pro-rata immediately-adjusted basis.

These are all predictable and standard transactional handling logics and requiring the user to write this logic and test it significantly slows down the launch time. WASP should come with an exhaustive, standard set of configuration options and administrative commerce UI functions to cover all standard commerce transactional scenarios.

A complete SaaS solution would also include pro-rata refund, pro-rata credit for downtime compensations, etc built into the admin dashboard

More in the Discord convo here: https://discord.com/channels/686873244791210014/1253446506489577492/1253446506489577492

We should explore these and see what of these makes sense to add to OpenSaas.
Some stuff we might want to add direclty, while some stuff might prove to be a bit too much for OpenSaas, making it overly complex. In that case, interesting alternative might be moving some of these more complex features into the Full Stack Module (FSM) (once we have them) that is specialized for this stuff.