#5027321 Billing: add membership subscriptions to purchase data

This adds membership subscriptions to the purchase list under /me/purchases.

Fixes #46604

To do:

• Add subscription site header with site icon (update: using a fallback icon for now and will address site icons in another PR)
• Update back button on membership detail page
• Update types

To test:

• On a site with membership subscriptions (ping me if you need one):
• Visit Me > Manage Purchase
• Verify that membership subscriptions are shown on the same screen (at the bottom) as your own site’s purchases
• Click on a membership subscription
• Verify that the subscription details page is unchanged
• Click “back”
• Verify that you’re taken back to /me/purchases (I’m not making this conditional, since we’ll be removing Other Sites as part of the next PR)