[IMP] web_editor, *: allow saving of snippets

* website, mass_mailing, website_event, website_form,
  website_mass_mailing, website_sale

Now the user can save snippets to use them on other pages.

task-2120409

closes odoo/odoo#40408

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