HG-230 Use Vignette for avatars

https://wikia-inc.atlassian.net/browse/HG-230
Created `WikiaUsers` component and `thumbnail` Handlebars' helper and used them to display top contributors of an article. This allowed to convert legacy avatar's URL to the Vignette's one.
remember that you can give back to community by creating a PR to DefinatellyTyped repo with fixes like these