feat: ability to unlink calendar event

[gikf]

• I have read Chapter's contributing guidelines.
• My pull request has a descriptive title (not a vague title like Update README.md).
• My pull request targets the main branch of Chapter.

Ref #2168

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• Adds ability to unlink calendar event after test.
• Test is a bit a lie. It doesn't check access to calendar event directly. It lists events from the chapter calendar (via the calendar_id), and checks, if calendar_event_id is among events for the chapter calendar. There are three possible results:
  • No matching event - calendar event either was deleted or (assumed) there's no access to it. Unlinking is possible.
  • Matching event - calendar event exists and there should be access to it, as we have access to chapter calendar. No unlinking.
  • Events can't be listed - this likely means there's no access to chapter. Right now this case is thrown together with other unexpected errors. No unlinking.
• I thought about just getting event instead. While it might prove complete lack of access, I'm unsure it could prove that chapter has full access to calendar event.

[ghost]

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[ojeytonwilliams]

LGTM 👍