Replace loading text by a spinner

templates/index.html

@@ -72,7 +72,7 @@
     let day_elem = document.getElementById('day')
     let result_elem = document.getElementById('result')
     document.getElementById('form').addEventListener('submit', () => {
-        result_elem.innerHTML = 'Chargement…'
+        result_elem.innerHTML = '<div class="spinner-border" role="status"><span class="visually-hidden">Chargement…</span></div>'
         fetch('/api/routes/' + day_elem.value + '/' + origin_elem.value + '/' + (destination_elem.value || 'undefined') + '/')
             .then(resp => resp.json())
             .then(routes => {