feat: First checkin for the ical direct features

[dploeger]

This implements a feature I called "ical direct" offering ical export and import features without the need of an external provider.

Currently, this is a draft where I'd like to know if you'd basically approve this and the way I'll implement it and whether you have some suggestions for the implementation.

Current state:

• Exporting an event (with currently only basic information) is the first implementation done (i18n wasn't yet done)

Ultimately, this will include:

• Exporting single events as well as a (optionall filtered) list of events
• Importing single events and complete calendars from ical