[FIX] web: fix kanban view progressbars related to records in another group (groupby:week)

* IMPACTED VERSIONS

  12.0+

* HOW TO REPRODUCE

locale :  Locale is en_US (or other SUNDAY based)
view:     CRM - My Pipeline - Kanban view
groupBy:  date_deadline:week (Expected closing)
records:  one record with a planned activity, on date_deadline = 2021-05-02 (SUNDAY)
          one record with no planned activity, on date_deadline = 2021-05-09 (SUNDAY)
remark:   don't keep any other record in MAY for better visibility

* PROBLEM

The progressbar of the week containing 2021-05-09 displays information about the record
from the week containing 2021-05-02

* CAUSE

1. PostgreSQL `date_trunc` function follows ISO8601 which essentially means that
  the start of a WEEK is always MONDAY. There is no argument to change this.

2. _read_group_format_result
  https://github.com/odoo/odoo/blob/27da86a138089c1838e4b94f8a6976995b9c1fff/odoo/models.py#L2210-L2219

  - Computes a label for a group of records.
  - Follows the locale for the label of the week, based on a date which is
    always a MONDAY because of `date_trunc`.

3. read_progress_bar
  https://github.com/odoo/odoo/blob/88957afca09662af7eaa19df1e40b3699e45e79e/addons/web/models/models.py#L167-L175



  - Associates a group label to a record.
  - Follows the locale for the label of the week, based on the date of a record
    which can be any day of the week. If the record is related to a SUNDAY and
    SUNDAY is the first day of the week, it would have been in a group with a
    different label in (2.) than in (3.) prior to this change.

* FIX

In 3., before associating a label to a record, we truncate the date to the
ISO start of the period, so that the label is determined for a record in the
same conditions than in 2. The locale is still used to get language-dependent
outputs with babel, but the grouping will always follows ISO8601 (date_trunc).

* TEST

Added a test for this problem case

TASK-ID : 2517848

closes odoo/odoo#69946

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>