[16.0][ADD] web_calendar_slot_duration_by_user

[imlopes]

[imlopes]

@yajo

Can you check this PR too ? Here my objective is to add a slot duration by user.

Context :

In some cases, an employee will plan by default meets each hour. But another employee (in another department) will plan his meets each 30min.

[imlopes]

@yajo this one too, can you check ? Thanks