The existing Monthly Convert modal only renders on the Ingenico ResultSwitcher. At least one required config value is only set in the ResultSwitcher, and the modal pops up as soon as the page is rendered in the browser
The new Adyen flow doesn't use a ResultSwitcher for most card payments - instead we do the whole payment while the user is on Special:AdyenCheckoutGateway and do a client-side redirect to the thank-you page when it's complete.
* Move the required config (thank you page URL and potentially, amount levels) to the parent GatewayPage setClientVariables.
* Render the modal on Special:AdyenCheckoutGateway but hide it in css
** Potentially include a property on the AdyenCheckoutGateway and IngenicoGatewayResult special page classes to indicate that donations can complete on that page as a flag to consult.
* Show the modal when the donation completes (in handleApiResult)