Add articleTypes URL param without UI

[MrOrz]

This PR adds a URL param, articleTypes, to rumors-site article list.

• Currently we do not officially launch image support, so the UI of this filter is not implemented yet.
  • Users must manually specify articleTypes in URL, it is not accessible from UI.
• articleTypes should be a comma separated list of ArticleTypeEnum (TEXT, IMAGE, VIDEO, AUDIO).
• When not given, articleTypes is not specified in ListArticles.
• If given, articleTypes is specified and thus would override MEDIA_ARTICLE_SUPPORT flag in rumors-api.

Screenshot

Only images

Only text

Not specified

The result depends on if the API has turned on MEDIA_ARTICLE_SUPPORT flag. On staging it is turned on, so it does not limit the article type to just text.

[coveralls]

Coverage remained the same at 75.831% when pulling e013012 on article-type into 1808d09 on master.