Add views for managing school years (including permissions, menu entry and form)

aleksis/core/forms.py

aleksis/core/menus.py

aleksis/core/rules.py

aleksis/core/tables.py

aleksis/core/templates/core/school_year/create.html

aleksis/core/templates/core/school_year/edit.html

aleksis/core/templates/core/school_year/list.html

aleksis/core/urls.py

aleksis/core/views.py