Rework donate_form.js to Display Amount Donated on Thank you page

[Rio517]

The fastest way to do this would be to store the amountDonated in the client when our pledge service responds with success, the fetch the value from this cookie.

https://github.com/MayOneUS/mayday-2.0-frontend/blob/master/_assets/javascripts/_includes/donate_form.js#L273
Current thanks page: https://mayday.us/donate-thanks/

The best way to do this would be to rework the pledge.js flow to allow a callback to be passed in that accepts the amount donated. This could fall back to the existing redirect.