Gradation: add route to add missing users

ui/src/routes/works/[wid]/+page.svelte

@@ -40,6 +40,11 @@
          stats.mean = sum / grades.length;
      });
  }
+
+ async function addMissingStudents(w) {
+     await w.addMissingGrades();
+     refresh_grades(w);
+ }
 </script>
 
 {#if $user && $user.is_admin}
@@ -72,12 +77,22 @@
                 {#if stats.mean > 0}(moyenne&nbsp;: {Math.round(stats.mean*100)/100}, min&nbsp;: {stats.min}, max&nbsp;: {stats.max}){/if}
             </small>
         </h3>
-        <button
-            class="btn btn-light"
-            on:click={() => refresh_grades(w)}
-        >
-            <i class="bi bi-arrow-clockwise"></i>
-        </button>
+        <div>
+            <button
+                class="btn btn-outline-info"
+                title="Ajouter les étudiants manquant"
+                on:click={() => addMissingStudents(w)}
+            >
+                <i class="bi bi-people"></i>
+            </button>
+            <button
+                class="btn btn-light"
+                title="Rafraîchir l'affichage des notes"
+                on:click={() => refresh_grades(w)}
+            >
+                <i class="bi bi-arrow-clockwise"></i>
+            </button>
+        </div>
     </div>
     <div class="card mt-3 mb-5">
         {#await gradesP}