coding_18847_delete_comment

- [x] code functions
   - [x] create function delete_comment (in controller)
   - [x] create function check_user (in model - use check permission )
   - [x] create function delete_comment (in model)
- [ ] unit testing
   - [ ] create function test_delete_comment (in controller)
   - [ ] create function test_delete_comment_notok(in controller)
   - [ ] create function test_check_user_ok (model)
   - [ ] create function test_check_user_notok (in model)
   - [ ] create function test_delete_comment_ok(in model)
We should use more meaning variable name such as "$deletable" instead if unclear name "check".