Configurable API rate limit

API rate limits ([as introduced here](https://framagit.org/les/gancio/-/issues/230)) are very useful to avoid someone creating too many events at once. But the current limits are too low for a fair use of the API. I want to use the API to automatically create a set of events (around 12). They are recurring events, but the dates are not periodic. Because of the limit, I need to wait 5 minutes every time before being able to use the API again. Some possible solutions/improvements that I can think of: - Have a configurable API rate limit editable from the UI. This would give the instance admins more freedom to set the limits they want for their API - Not allow anonymous calls to the API, and always require an API_KEY to be sent, allowing higher limits