Fix the PDP subscription form

[vassalloandrea]

The PDP subscription form is not working correctly:

1. it appears also if the product is not subscribable
2. the default PDP quantity input is used to set the subscribable_id (the variant id)
3. the interval units fields are not filled with a default value
4. the subscription quantity is handled by a redundant field

How to fix:

1. show the subscription form only if the product or one of the product variant is subscribable
2. use the default PDP quantity input to set the subscription quantity
3. fill the interval units fields with a default value
4. remove the redundant field

[stale]

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