Add timezone to meeting

[Elblinator]

A meeting should have in future a timezone which is configurable while creating the meeting but not changable after the meeting is created.

a payload from the client could be

[
    {
        "action": "meeting.create",
        "data": [
            {
                "committee_id": 1,
                "start_time": 1772319600,
                "end_time": 1772924400,
                "time_zone": "Europe/Berlin"
                "name": "Mitgliederversammlung (7)",
                "location": "",
                "admin_ids": [
                    3
                ],
                "organization_tag_ids": [],
                "external_id": "",
                "language": "de",
                "set_as_template": false
            }
        ]
    }
]