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
   - [x] create function test_delete_comment_ok (in controller)
   - [x] create function test_delete_comment_notok(in controller)
   - [x] create function test_is_deletable_notok(in controller)
   - [x] create function test_is_deletable_ok (model)
   - [x] create function test_is_deletable_notok (in model)
   - [x] create function test_delete_comment_ok(in model)
   - [x] create function test_delete_comment_notok(in model)
Research and use "bind variables" to prevent from SQL injection.