coding_18843_add_comment

 - [ ] code functions
    - [ ] post_comment (controller)
    - [ ] create_comment (model)
- [ ] code unit test
    - [ ] test_post_comment_ok (controller)
    - [ ] test_post_comment_notok (controller)
    - [ ] test_post_comment_ok (model)
    - [ ] test_post_comment_notok (model)