#7025723 Purchases: Add notice that purchases may use backup if available

Changes proposed in this Pull Request

This adds small text notices to the purchase list and details pages if there is a backup payment method set.

Each notice is a link to the payments documentation on backup cards (similar to the links that are added by #56665). Note that the actual updated docs are not yet published, but the link should still work.

Screenshots

The subscription list page:

The subscription details page:

Testing instructions

• Make sure that no payment methods are marked as backup (you can use Payments Admin).
• Visit /me/purchases and verify that you do not see any notice of backup payment methods.
• Click on a purchase and verify that you do not see any notice of backup payment methods.

Then:

• Assign at least one payment method as a backup.
• Repeat the above steps and verify that you see a notice on both pages.