prevent /stripe/create-event from being called twice and throwing a misleading error message

[charleslavon]

Address this TODO

Originally posted by @shelegdmitriy in #52 (comment)

[shelegdmitriy]

@charleslavon I grab this issue because I think it's related to this task and related to the issue mentioned by Caleb

[calebjacob]

Copying my comment from the PR here just to make it easier to find:

This would be a great chance to refactor any of our form submission API calls to use useMutation provided by React Query: https://tanstack.com/query/v4/docs/framework/react/guides/mutations

This will give us many benefits like loading/error states and will make sure duplicate requests don't occur.