Original PR description

Steps to reproduce:
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1. Install Planning app
2. Have an employee A with 40h/week working schedule and an employee B with 35h/week working schedule
3. Create a shift and assign it to employee A, the default allocated hours should be 8h
4. From the Gantt view, duplicate this shift and assign it to employee B (use drag and drop) 
5. Problem: The allocated hours of the duplicated shift are not recomputed, they are equal to 8h instead of 7h

Fix:
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When duplicating a shift and assigning it to another resource, the copy() method is called with the resource_id of the target resource (an employee in this case). But _compute_allocated_hours() is not called so we have to call it explicitly in this case.

task-3978590