Apple Pay improvements

[adavidw]

I'd love someday to get a few Apple Pay improvements:

• Review for compliance with everything in the Apple interface and style guidelines.
• Error handling to display appropriate messages in cases where Apple Pay can't be used.
• When used on a system that's capable of Apple Pay but isn't currently set up, display the "Set up Apple Pay" button.

Not high priority, but thought I'd log an issue as a reminder for me to look into these someday.