[REF] website_forum: mass moderation, modern UI bs4 and clean type

Now forum only allow to post message of type 'question'.
New 'modern' UI with Bootstrap 4
New moderation modal for bulk spam

Co-authored-by: qha <qha@odoo.com>
Co-authored-by: qsm <qsm@odoo.com>
Co-authored-by: jke <jke@odoo.com>

Thanks to @qha-odoo for UI
Thanks to @qsm-odoo for reviewSsss

closes odoo/odoo#29235