Solidus Subscriptions API allows to create subscriptions of not subscribable products

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Substantially all combinations of products and variants can be purchased as subscription despite the subscription flag being inactive on parent product or variants.

Solidus Version:
Current
To Reproduce
Install the sub gem and try to subscribe to non subscribable products.

Current behavior
You can subscribe to non subscribable products and variants.

Expected behavior
You shouldn't be able to do that.

Additional context

[fthobe]

Closed by #312