Integrate add content page into backend

The page can be found under `/add`.

src/main/resources/static/add.js

@@ -46,11 +46,14 @@ form.addEventListener("submit", e => {
     fetch("/api/v1/datasets", {
         method: "POST",
         body: JSON.stringify(newContent),
+        headers: {
+            "Content-Type": "application/json;charset=utf-8"
+        }
     }).then(response => {
         if (response.status == 200) {
             location.assign("/");
         } else {
-            addBtn.disabled = !checkValidity();
+            addBtn.disabled = !form.checkValidity();
         }
     });
 });