Simplify "Register absence" by preselecting persons and add links

aleksis/apps/alsijil/forms.py

aleksis/apps/alsijil/menus.py

aleksis/apps/alsijil/rules.py

...sis/apps/alsijil/templates/alsijil/absences/register.html

...apps/alsijil/templates/alsijil/class_register/person.html

...s/alsijil/templates/alsijil/class_register/week_view.html

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

aleksis/apps/alsijil/urls.py

aleksis/apps/alsijil/views.py