Fix tests after removing the index action

The show action makes more sense in the controller. Also, the filter
scoped was failing due to Rails raising on missing actions (because we
have deleted the index action). Inlining the concern fixes it and clarifies
where this action is implemented.

test/controllers/users_controller_test.rb

@@ -1,14 +1,6 @@
 require "test_helper"
 
 class UsersControllerTest < ActionDispatch::IntegrationTest
-  test "index" do
-    sign_in_as :kevin
-
-    get users_path
-    assert_in_body users(:david).name
-    assert_in_body users(:kevin).name
-  end
-
   test "new" do
     get new_user_path(params: { join_code: "bad" })
     assert_response :forbidden