Fix checkout process

[Gnito]

Issues:

• Submit button was enabled prematurely for onetime payments.
  It didn't check if stripeElements aka card number field was filled.
• Changing from onetime payment to default card was not working correctly in case of error (e.g. network error).
• Calling stripe.confirmCardPayment when status is requires_capture is unnecessary.

  400 "You cannot confirm this PaymentIntent because it has a status of requires_capture. Only a PaymentIntent with one of the following statuses may be confirmed: requires_confirmation, requires_action."

Note: these are cherry-picked commits from FTW-product:

• PR 111

• PR 120

• PR 121

  (Changelog updates were not included)

[kpuputti]

I don't fully understand the Stripe/PI flow, but the code looks clean in general.