Create event with JSON object

[keabard]

It would be cool to avoid hosting our JSON or giving public access to an endpoint, and just paste a JSON object to configure a new OpenFeedback event.

This PR is meant to do that.

Could not manage to make the Firebase install step work tho, so this feature is not tested at all. I hope not much more is needed and that you will be able to complete it 🙏

https://github.com/HugoGresse/open-feedback/pull/1654