ICalendar file export for events

[OverkillGuy]

Fixes #53.

• New dependency "icalendar" to export events into ical format.
• Convert event.py Events into ICalendar events
• Completely untested (I don't have events sample to play with)
• Gotta check/recheck the timezone management on that code (does no tz info mean UTC?)

I'm fairly sure that uploading an ical file like that, to an HTTP endpoint, is sufficient to make the famed "webcal feed", so users can import them as readonly.

[OverkillGuy]

Should make it more clear: this PR is a good first step to fully customizable per-user calendar exports: Current function gives a ical content from a given list of event, can be plugged in to per-user filtering system, returning the list of event that matches player preferences.

[Gehock]

I tried running the code but there seem to be some syntax errors in there (plus the EventDatabase doesn't currently support loading the events without being connected to Discord, which should be fixed). Tried to get the Actions to run by reopening the PR but apparently they need some extra configuring as well