VOLDEV-2 Add user right for being allowed to add article comments

[VOLDEV-2](https://wikia-inc.atlassian.net/browse/VOLDEV-2)
@Grunny @michalroszka 
Output format should be `escaped()`. You can also get rid of the explicit formatNum call and make use of `numParams` (i.e. `wfMessage( 'oasis-comments-header' )->numParams( $countCommentsNested )->escaped()`).