Add nesting for "Users and Roles" admin component

Now instead of a top level Users component, the main landing page is
"Users and Roles" (with the users page being the pre-selected tab, so
the only visual change is a new page header and a new tab component to
swap between "Users" and "Roles".

admin/app/components/solidus_admin/users/index/component.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-class SolidusAdmin::Users::Index::Component < SolidusAdmin::UI::Pages::Index::Component
+class SolidusAdmin::Users::Index::Component < SolidusAdmin::UsersAndRoles::Component
   def model_class
     Spree.user_class
   end

admin/app/components/solidus_admin/users_and_roles/component.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class SolidusAdmin::UsersAndRoles::Component < SolidusAdmin::UI::Pages::Index::Component
+  def title
+    page_header_title safe_join([
+      tag.div(t(".title")),
+    ])
+  end
+
+  def tabs
+    [
+      {
+        text: Spree.user_class.model_name.human(count: 2),
+        href: solidus_admin.users_path,
+        current: model_class == Spree.user_class,
+      },
+    ]
+  end
+end

admin/app/components/solidus_admin/users_and_roles/component.yml

@@ -0,0 +1,2 @@
+en:
+  title: "Users and Roles"

admin/spec/features/users_spec.rb

@@ -11,6 +11,7 @@
     create(:user, :with_orders, email: "customer-with-order@example.com")
 
     visit "/admin/users"
+    expect(page).to have_content("Users and Roles")
     expect(page).to have_content("customer@example.com")
     expect(page).not_to have_content("admin-2@example.com")
     click_on "Admins"