Hr Contract Employee Calendar Planning

This module ensures a consistent working times history when using the
**hr_contract** and **hr_employee_calendar_planning** modules together.

There are 3 different data models which can relate to working time
(resource.calendar) records:

- Employees (hr.employee)
- Contracts (hr.contract)
- Resources (resource.resource) -\> related to hr.employee through
  resource.mixin

The **hr_employee_calendar_planning** module adds the calendar_ids field
to employees, which allows a more flexible working times configuration:
Instead of selecting a single resource.calendar, multiple calendars can
be combined into one auto-generated calendar.

However, contracts are not considered when creating auto-generated
calendars. This can lead to unexpected behaviour, because the active
contract calendar and the employee calendar can diverge (calendar
mismatch). Additionally, when configuring a new contract, or changing
the existing contract calendar, the employee calendar will be
overwritten by the contract calendar.

To resolve this issue, this module migrates current and previous
contract calendars into the calendar_ids and thus the auto-generated
calendar. Additionally, changes to the employee calendar by contracts
are prevented. The resource_calendar_id field is hidden in contract
views, since all working times should be managed in the calendar_ids
field for each employee. However, it is still possible to keep a
meaningful contract history, e.g. for salary information.