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.
how about caching `Mercury.Modules.Thumbnailer`
I think it would look cleaner here