Create an updatable calendar for every floor

[OmTheTurtle]

As I mentioned it in #343 it would be a nice feature to create a calendar for floors that the users can import to e.g. Google Calendar that will update every time the someone adds a new event to the given floor. After a quick search it turns out you just have to serve an .ics file from a server and then it can be imported via a URL (Neptun does the same thing).

Since we run in Docker in prod, we should find a way to store these file outside the container, otherwise they'll be deleted every time we redeploy our app.

[nmate980829]

If we use the ical-generator package, serving the file won't be a problem. We can use the ics package too (this way we have to serve the file).