Add subtasks for tasks on backend

[biggujo]

Subtasks:

• subtasks has the same scheme as the tasks;
• Task model has many Tasks;
• make work possible with the following routes:
  • private tasks of the user:
    • GET /api/tasks/:id/subtasks - get all subtasks;
    • POST /api/tasks/:id/subtasks - add a subtask;
    • PUT /api/tasks/:taskid/subtasks/:subtaskid - update a subtask;
    • DELETE /api/tasks/:taskid/subtasks/:subtaskid - delete a subtask;
  • group tasks:
    • GET /api/groups/:groupid/tasks/:id/subtasks - get all subtasks;
    • POST /api/groups/:groupid/tasks/:id/subtasks - add a subtask;
    • PUT /api/groups/:groupid/tasks/:taskid/subtasks/:subtaskid - update a subtask;
    • DELETE /api/groups/:groupid/tasks/:taskid/subtasks/:subtaskid - delete a subtask

Hint: maybe I've made a mistake in routes here, but the main point must be clear