[15.0][IMP] hr_contract_employee_calendar_planning

[jonasbuchholz]

Improve hr_contract_employee_calendar_planning:

• hide resource_calendar_id in hr_contract_history view
• make sure that creating new contracts does not overwrite the auto-generated calendar
• the post-install hook should only crop the first calendar if there are multiple overlaps

[github-actions]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.