Detect when we are running on a WooCommerce Subscriptions staging site

[daledupreez]

Fixes STRIPE-751
Related to #4131

Changes proposed in this Pull Request:

This PR detects when we are running on WooCommerce Subscriptions staging sites as a way to detect whether we should detach customers from payment methods. This expands on our previous logic that only looked at the return value from wp_get_environment_type(). This PR does not implement a full fix for the issues flagged in #4131, but it does mean that staging sites using WooCommerce Subscriptions will behave better.

Testing instructions

TBD

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
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