Move PaymentSheet example to /checkout endpoint.

[brnunes-stripe]

Summary

Use /checkout endpoint for PaymentSheet example, as implemented in Glitch example.

Motivation

Simpler integration, use same endpoint as iOS.

Testing

• Added tests
• Modified tests
• Manually verified

[brnunes-stripe]

Will wait until example backend implements a /checkout endpoint like the Glitch example, so the example app uses a single backend.

[michelleb-stripe]

Let's just add a second property for the moment until we decide how/if we want to split the example app. This will allow us to close out this PR, start moving off of heroku, and take advantage of returning/new customers - woohoo!

[brnunes-stripe]

Let's just add a second property for the moment until we decide how/if we want to split the example app. This will allow us to close out this PR, start moving off of heroku, and take advantage of returning/new customers - woohoo!

This is not great, but it's the simplest way if we want to merge this. We should refactor ASAP.
7b3608e

[brnunes-stripe]

Merging this into mc branch then will work on top of it to move the PaymentSheet example into its own app.