Use new alert syntax

.../alsijil/templates/alsijil/absences/register_confirm.html

...s/apps/alsijil/templates/alsijil/excuse_type/warning.html

...templates/alsijil/group_role/partials/assigned_roles.html

...is/apps/alsijil/templates/alsijil/group_role/warning.html

...lsijil/templates/alsijil/partials/persons_with_stats.html