Draft: [#3052574] Facets AJAX

Combine all of the compatible contributions into a working branch to allow easy review and continued work on the issue.

Please "fix conflicts+rebase 2.0.x" instead of merging in 2.0.x to avoid breaking history to a point that automated rebase no longer works.

src/Controller/FacetBlockAjaxController.php

@@ -141,6 +141,12 @@ class FacetBlockAjaxController extends ControllerBase {
     $new_request = Request::create($path);
     $request_stack = new RequestStack();
 
+    // Add session to the response if set.
+    if ($request->hasSession()) {
+      $request->getSession()->save();
+      $new_request->setSession($request->getSession());
+    }
+
     $processed = $this->pathProcessor->processInbound($path, $new_request);
     $processed_request = Request::create($processed);