feat: Translations for grid related menu items

The new main menu items were added to the MAIN_MENU list in local_settings.py, like this:

MAIN_MENU = [
    {"url": "/forecasts", "label": "Forecasts"},
    {
        "label": _("Risk maps"),
        "child_items": [
            {
                "url": "/spatial",
                "label": _("About risk maps"),
            },
            {
                "url": "/spatial/gridmap/SEPTREFHUM/",
                "label": _("Septoria reference humidity model"),
            }
        ],
    },
    {"url": "/observations", "label": "Observations"},
    {"url": "/messages", "label": "Messages"},
]

The messages were generated and compiled as follows (starting from the root of the project):

cd VIPSWeb
../manage.py makemessages -a
(add Norwegian translations to po file)
../manage.py compilemessages