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.
Took it from https://github.com/Wikia/mercury/blob/master/public/scripts/main/components/CollapsibleMenuComponent.ts#L9

Should I change them both to `classes`?