Create Current model as attributes singleton

This uses `ActiveSupport::CurrentAttributes` to track the current user
across the request.

app/controllers/concerns/authentication.rb

@@ -1,10 +1,17 @@
-# Requires a user to be logged in through GDS SSO for any action.
+# Requires a user to be logged in through GDS SSO for any action, and keeps track of the current
+# user through `Current`.
 module Authentication
   extend ActiveSupport::Concern
 
   included do
     include GDS::SSO::ControllerMethods
 
-    before_action :authenticate_user!
+    before_action :authenticate_user!, :set_current
+  end
+
+private
+
+  def set_current
+    Current.user = current_user
   end
 end

app/controllers/recommended_links_controller.rb

@@ -59,6 +59,6 @@ def set_recommended_link
   def recommended_link_params
     params
       .expect(recommended_link: %i[link title description keywords comment])
-      .merge(user_id: current_user.id)
+      .merge(user_id: Current.user.id)
   end
 end

app/helpers/application_helper.rb

@@ -1,6 +1,6 @@
 module ApplicationHelper
   def navigation_items
-    return [] unless current_user
+    return [] unless Current.user
 
     [
       {
@@ -9,7 +9,7 @@ def navigation_items
         active: controller.controller_name == "recommended_links",
       },
       {
-        text: current_user.name,
+        text: Current.user.name,
         href: Plek.new.external_url_for("signon"),
       },
       {

app/models/current.rb

@@ -0,0 +1,3 @@
+class Current < ActiveSupport::CurrentAttributes
+  attribute :user
+end