feat/add ical endpoints

[GuillaumeDecMeetsMore]

Changed

• Add new ical endpoints, allowing to get ical definitions of the events for a time range
  • Endpoint for the user himself
  • Endpoint for admin access
• Nothing public so far, but we will need to add some way to expose it publicly at some point

Notes

• It's not fully correct/implemented yet, but as the PR is already super big, I'm splitting the work
  • TODOs
    • properly fetch all the recurring events that impact the range
    • Add a limit on the number of events, and the possibility to fetch more (e.g. by providing an offset)
    • Likely later, but allow to expose publicly the ical (on another, harder to guess, URL + proper rate limit to avoid brute force)