[18.0] [FIX] hr_timesheet_begin_end: Validate 'overlap' on employee instead of user

[bertstomphorst]

The validation for overlapping times was done on user_id, which is the user entering the lines. This should be on employee_id, the employee for who the times are entered. This is relevant when entering times within a Task or within 'All Timesheets'

@guewen @luismontalba @ernestotejeda

[bizzappdev]

@bertstomphorst, Thank you for the fix. Can you please squash the commits?

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖