First Pull request for code review - Registration, User Log in, Pagination

In my opinion, this method should stay where it is. This meant that you're getting the comments from a specific thread. In this change, the way of getting comments became like this on this commit : https://github.com/fpambid/Board_Exercises/commit/8f261653e59d46a7bb7567bce4255934b4ea711c#diff-6a95776461d3100924681b80e5d56588L6

```php
$thread = Comment:get(Param::get('thread_id));
$comments = $thread->getComments();
```
which is logically incorrect since you're getting the comments from Comment, compared to before : 
```php
$thread = Thread::get(Param::get('thread_id'));
$comments = $thread->getComments();
```
which gets the comments from Thread