[FIX] web: display correct week number in calendar views

To compute the week number, the fullcalendar.js library uses the locale.
It depends on the day on which the week starts (usually Sunday in the Americas,
Monday in Europe), as well as which week is considered the first of the year.
In commit:
https://github.com/odoo/odoo/commit/717b472ef9ad6d2998cbefb4aafe2d1199e6235d
the starting day was explicitly added, however it was computed as isoWeekday.
The ISO first day of the week is always on Monday.
It resulted that if the locale used Sunday as first day of the week, the
calendar would consider it to be of the previous week, and thus displayed
(weeknumber - 1).
The fix does the same thing as was done in v10, taking the locale into account:
https://github.com/odoo/odoo/commit/df124080b624a8bdb86fec352fe690725ed36bac

opw 1853745