Issue occurs due to the way we compute the default value in base ir ui view.
The method _compute_defaults don't set extension mode under some condition
even if we provide an inherit_id view.

This commit double fix it, we use now self.env['ir.ui.view'] to create the
new view and so always compute the mode based on 'if inherit id or not'.
Second fixes is to explicitely set mode manually as 'extension'.

before commit:
when you drop snippet to blog sidebar and click save button.
The changes of user is not visible in sidebar because view created in mode
'primary' and not 'extension'

task-2311520

closes odoo/odoo#65182

X-original-commit: a944ce9b
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: jpr-odoo <jpr@openerp.com>
Co-authored-by: Jairo Llopis <jairo.llopis@tecnativa.com>