[FIX] web_tour: only allow one active tip at once
[FIX] web_tour: only allow one active tip at once

Before this commit, the code for the tour system was kind of naive and
simply activated tips for each tour.  This was not really an issue,
since most user install one app at a time, so they had the time to
follow a tour, and when they install the next app, it is no longer
active.

However, since we activated the multi-app feature in our website, it now
happens that multiple apps are installed at once.  Because of this, this
commit is now necessary.

closes odoo/odoo#28178
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo