Add an API for Event creation, modification & deletion

As discussed on Whatsapp, this is the first step of the API requested by Christian. It allows us to do the following:

• GetEvents GET: gets all the event of the current User, maybe with a flag for future Events only, or unreported events from the past or similar CreateEvent POST: create a new event

In addition, it allows someone to edit the event, or to delete it from the API, if the use case is there.

I will implement result upload via API in a follow up MR.