[FIX] web: Avoid blinking attachment box

Go to hr.applicant view form, Edit and change something.
Each time you change a field, the Attachment box in the chatter is toggled
(as if you were clicking on the small attachment button each time).
This a wrong behavior of the `open_attachments` option of the `mail_thread`
widget.
Opening the attachment box should be done at the widget initialisation,
not at every render.

closes odoo/odoo#38519

Signed-off-by: lul-odoo <LucasLefevre@users.noreply.github.com>