This commit slightly adapts the survey navigation buttons to make sure that
they are always positioned at the same spot.
Before this change, if you rapidly click the "back" button a lot of times, when
it disappears, you will misclick the "Odoo" button and land on the Odoo
website, which can be very annoying during a real test with a timer.
Now, we disable the button when it becomes non-clickable instead of removing
it, so that it always stays at the same spot.
Task-2665998
closesodoo/odoo#78513
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>