[FIX] website, *: force website UI dropdown style

* web_editor

The purpose is to force our dropdown style in our UI so that themes do
not break it anymore (same way all the other elements are forced (cards,
headings, etc)).

task-2059538

closes odoo/odoo#38268

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: qsm-odoo <qsm@odoo.com>