[IMP] website_crm_partner_assign: fix multi datepickers on portal

When a portal user is editing opportunity from front-end, there are
two date inputs, 'Expected Cloing' and 'Next Activity Date'. But both
of them have two pickers for selecting date, a native one and a
bootstrap one, which was probably side effect of commit[1].

With this commit, we only keep the bootstrap picker so that it
respects the date format based on language, and for the check,
to make sure it doesnt break the backend (see ticket from commit[1]
for more details), we simply consider the date with year less than
1900 as invalid.

We also provide a bit more space for bootstrap datpicker to open while
setting the closing date, so that the scrollbar is not introduced on
the modal. And for the next activity date, we make sure that the
picker is opened always on top of the input (which is not the case
currenlty in some screensizes, sometimes it is opened below the input
so it is hidden under the extra scroll introduced with picker).

commit[1] - https://github.com/odoo/odoo/commit/b2b639b693dd7215e3ee501ac89c35f3a6b88bb5



TaskID-2624728

closes odoo/odoo#75453

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