Show confirmation upon successfull enrollment in Free Connector Program and verification email re-send

[ambirdsall]

Part of #21074. The stripe successUrl appends a specific query param to the current URL, so the enrollment banner components which serve as the enrollment entry points (and are therefore always part of the successUrl's UI) can check for that query param on page load; if present, they will display a success Toast and avoid rendering the banner UI.

Similarly, there should be a Toast triggered when the request to firebase to re-send the address validation email returns successfully.