HG-97: JSSnippets return empty string on ArticleAsJson

Extensions frequently use `JSSnippet` to load resources on per-page basis. This is generally great approach instead of the case when we want to use the resulting page as a content in single page app. In this case having  `<script>` tags is highly undesirable.

The purpose of this PR is to suppress the `JSSnippet` tegerated `<script>` tags, when the Article is requested as JSON by the Mercury API. :postal_horn: 

/cc @macbre @hakubo