change "delete task" button from blue to red, scary color for scary button.

templates/task.html

@@ -21,7 +21,7 @@
                 <form method="POST"
                       action="{{url_for('delete_task',
                                 task_id=task.id) }}">
-                    <input class="btn btn-primary" type="submit" value="Delete Task"
+                    <input class="btn btn-danger" type="submit" value="Delete Task"
                            onclick="return confirm('Are you sure you want to delete this task?')">
                 </form>
             </div>