Modified user information API

* I added HTTP 409 as the return code for `POST` requests to
  `/api/users`, and `PUT` and `PATCH` requests to `/api/users/me`.
* I added HTTP 404 as the return code for `PUT` and `PATCH` requests
  to `/api/users/me`.
* I changed `DELETE /api/users/me` to `POST /api/users/me/delete`.
* I added `GET /api/logout`.
Force `PATCH` method for editing any object

* For editing an object of any kind, `PATCH` method rather than `PUT`
  method should be used.