[FIX] website: prevent error on edit mode save for restricted editor

- Before this commit, When user with restricted rights
tries to edit frontend page and without changing anything click
on save button generate traceback due to invalid return type.

- After this commit, On hitting save button on restrict mode resolve
the traceback by changing function return type to promise.

task-2379101

closes odoo/odoo#66357

Signed-off-by: Romain Derie (rde) <rde@odoo.com>