Changing suggested donation with keyboard doesn't update amount #438

1. Go to https://www.documentcloud.org/donate
2. Note that $50 is selected
3. Click on $100 and click Donate!
4. Enter dummy contact info in first Stripe screen
5. On second Stripe screen (payment), ✅ note that the payment button reads $100
6. Refresh page to reset form
7. Using keyboard only, tab or arrow left or right to select another amount ($25, $100, or $250) and click Donate!
8. Enter dummy contact info again
9. On payment screen, ❌ note that the payment button still reads $50

We aren't binding to sufficient events. This'd be easier if Bootstrap had a better API for listening for button group changes. Of note: if the user tabs to the Other amount and types in something else, that does work.