Style warning about mentioning many people in a comment

Fixes #2885 

* Create issue:

    Before:

    ![Screenshot_2015-12-03_15.23.44](/uploads/6da9b326e47dc5e8c127a581af99403c/Screenshot_2015-12-03_15.23.44.png)

    After:

    ![Screenshot_2015-12-03_15.21.29](/uploads/bbe328d30a32d642c280bf2ad6d2832c/Screenshot_2015-12-03_15.21.29.png)  
      

* New merge request:

    Before:

    ![Screenshot_2015-12-03_15.24.06](/uploads/79d6f8f52ba2753aaecf6b6bbd04f0d0/Screenshot_2015-12-03_15.24.06.png)

    After:

    ![Screenshot_2015-12-03_15.21.48](/uploads/f01ea8e7d058a9f8b803ca1ce4f533ad/Screenshot_2015-12-03_15.21.48.png)
     

* New comment in a merge request:

    Before:

    ![Screenshot_2015-12-03_15.11.23](/uploads/5d958375812d21d41e00694186c7ef7a/Screenshot_2015-12-03_15.11.23.png)

    After:

    ![Screenshot_2015-12-03_15.09.08](/uploads/3d6bde6aad46c7d9e53b70a47eecf7c0/Screenshot_2015-12-03_15.09.08.png)
     

* New diff comment:

    Before:

    ![Screenshot_2015-12-03_15.25.50](/uploads/08ac9c261c1aa9693706da714c121e15/Screenshot_2015-12-03_15.25.50.png)

    After:

    ![Screenshot_2015-12-03_15.46.20](/uploads/07a8b8708ad825eb8cda7e5e40116b81/Screenshot_2015-12-03_15.46.20.png)

See merge request !1971