Allow site admins to renew plans (even if they do not own it)

[mrfoxtalbot]

What

Site admins should be able to renew any subscriptions on their site even if the subscription was added by a different user.

This plan was purchased by a different WordPress.com account. To manage this plan, log in to that account or contact the account owner.

Why

We receive a non-insignificant number of support requests from users who are administrators of a website but do not own the subscriptions for that site. The subscriptions were purchased by a different user. This is a fairly common scenario in organizations ran by volunteers where people come and go.

In cases like this, we will create a manual payment for the non-owner admin and, once they complete the payment, we will update the subscription's expiration date. This poses several problems:

• It's labour-intensive and can lead to errors.
• Some errors can lead to AR issues.
• Users will be in the same scenario 1 year down the road.
• We are losing revenue from users who fail to contact us or follow the manual payment link.labor-intensive

How

We already have a "Gift a Subscription" feature that allows ANYONE to pay for the plan renewal as long as the site's admin activates it. The net effect would be similar to showing this banner to non-owner admins on the site. I am not proposing that specific path, it is just an illustration of the idea.

We could re-use this flow to let admins pay for renewals with two caveats:

• Payments would be reflected in the non-owner admin's SA to avoid AR loopholes and to allow for cancelations & refunds.
• The ownership of the subscription would not change.

This would also fix https://github.com/Automattic/wp-calypso/issues/46706 already have