coding_18843_add_comment

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