Improve Donations reCaptcha integration.

[carltongibson]

Follow-up from #935.

Current limitations:

• Ideally, the submit button would be disabled prior to the initial client-side recaptcha check completing, thus making the required response value available.
• If the user closes the Stripe form, they need to reload to acquire a new recaptcha response value. (Ideally, I guess we refresh the value on the Stripe handler.close() event.)

[stale]

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[carltongibson]

This no longer applies since we moved the payments to hosted checkout page. (Refs #1052.)