* Bucket and sample on server by using the
`WikimediaEvents.WebABTestArticleIdFactory` service from
WikimediaEvents (soft dependency)
* Add linkHijack.js so that users bucketed in one group have the
possibility of remaining in that group if they click a link to another
page.
Bug: T302046
Depends-On: Ie6627de98effb3d37a3bedda5023d08af319837f
Change-Id: Iff231a976c473217b0fa4da1aa9a8d1c2a1a19f2