add nonce element

CTFd/templates/admin/login.html

@@ -71,6 +71,7 @@
                         <button type="submit" id="submit" tabindex="5" class="btn btn-md btn-theme btn-outlined pull-right">Submit</button>
                     </div>
                 </div>
+                <input type="hidden" name="nonce" value="{{ nonce }}">
             </form>
         </div>
     </div>