Purchases: Handle asynchronous plan removals/cancellations from Atomic sites #6640640

Fixes #55194

Changes proposed in this Pull Request

Implements a basic proof of concept that handles the asynchronous removals/cancellations of plans from Atomic sites.

The PoC lives behind the atomic/automated-revert feature flag which is disabled by default in all environments.

Testing instructions

• Use the Calypso live link below.
• Switch to an Atomic site.
• Go to Upgrades > Purchases.
• Select the site plan.
• Try removing/cancelling it.
• Make sure you are asked to contact support in order to remove/cancel it.
• Append ?flags=atomic/automated-revert at the end of the URL and reload.
• Go through the removal/cancellation flow.
• Make sure you get a success notification indicating the removal/cancellation has been queued.
• Make sure you cannot remove/cancel the plan again (the button shouldn’t show up).
• Manually run the atomic_subscription_revert_after_lossless_import async job (see testing instructions in D66763-code).
• Reload Calypso.
• Make sure the Atomic site is on a Free plan now.
• Make sure there are no regression when removing/cancelling a plan from a Simple site (the success notification should denote the plan has been immediately removed/cancelled).