T250851 enabled the client search experience to be swapped in Core. T257647 makes the new client experience available. This task captures the work to add a configuration that swaps between the old and the new.
== Acceptance criteria
-  Regardless of configuration, Legacy mode operates the same.
-  The flag must be overridable by URI query parameter so that we can QA Search on the Beta Cluster and the initial deployment wikis.
-  T249297 is revised to include deployment instructions and a patch (-2'd initially).