When no related articles have been specified by an editor we instead
hit request pages similar to the current page using the CirrusSearch
extension's "morelike:" feature [0].
Changes:
* Config variable introduced RelatedArticlesUseCirrusSearch which allows
you to turn on use of the CirrusSearch API.
* Introduce a RelatedPagesGateway for dealing with making the API call
and returning consistent results
* Move the "simple" API call for hydrating related pages fetched from
the wgRelatedArticles configuration variable into RelatedPagesGateway
* Reduce the bootstrap module to just a bootstrap module!
Bug: T116707
Change-Id: Ia0ced1d7ae57c0939d1f5af275aa9d393f1420b1
