DEV: Setup experimental sidebar skeleton (#16575)

* hidden siteSetting to enable experimental sidebar
* user preference to enable experimental sidebar
* `experimental_sidebar_enabled` attribute for current user
* Empty glimmer component for Sidebar

spec/requests/users_controller_spec.rb

@@ -2309,6 +2309,21 @@ describe UsersController do
           expect(response).to be_forbidden
           expect(user.reload.name).not_to eq 'Jim Tom'
         end
+
+        context 'enabling experimental sidebar' do
+          before do
+            sign_in(user)
+          end
+
+          it "should be able to update UserOption#enable_experimental_sidebar" do
+            SiteSetting.enable_experimental_sidebar = true
+
+            put "/u/#{user.username}.json", params: { enable_experimental_sidebar: 'true' }
+
+            expect(response.status).to eq(200)
+            expect(user.user_option.enable_experimental_sidebar).to eq(true)
+          end
+        end
       end
     end
 
@@ -3882,6 +3897,23 @@ describe UsersController do
           expect(topic_post_count[topic.id.to_s]).to eq(2)
         end
       end
+
+      it "includes UserOption#enable_experimental_sidebar when SiteSetting.enable_experimental_sidebar is true" do
+        SiteSetting.enable_experimental_sidebar = true
+        user1.user_option.update!(enable_experimental_sidebar: true)
+
+        get "/u/#{user1.username}.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["user"]["user_option"]["enable_experimental_sidebar"]).to eq(true)
+      end
+
+      it "does not include UserOption#enable_experimental_sidebar when SiteSetting.enable_experimental_sidebar is false" do
+        get "/u/#{user1.username}.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["user"]["user_option"]["enable_experimental_sidebar"]).to eq(nil)
+      end
     end
 
     it "should be able to view a user" do