Add ical subscription

[david-venhoff]

Short description

This pr implements support for automatically creating events from an icalendar. This only implements step 1 of issue #100, as it is not currently possible to automatically use machine translation with this feature and we need another pr for that. However, it is possible to manually start machine translation for imported events.

For now, this feature is only visible to service team members and cms team members.

Also, this pr is currently a draft because it is blocked on #2891 ~~and #2850~~, but it can already be reviewed.

To test this pr, you could use this ical url, which includes events for july from Saarburg: https://www.saarburg.de/?post_type=tribe_events&tribe-bar-date=2024-07-08&ical=1&eventDisplay=list

Proposed changes

• Add a new model for external calendars, which contain an url to the ical file
• Add fields to the event model to specify whether this event was automatically imported, and link it to the icalendar
• Implement functionality for converting icalendar events into cms events and event translations
• Add UI for managing external calendars

Side effects

• We need a new cronjob that runs the import events command every once in a while

Resolved issues

Fixes: #100

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