users: lock out users with invalid names until they change their name.

Forcibly redirect users to the name change page if their name is
invalid. This means user with invalid names can't do anything or view
any pages until they change their name. API requests are still allowed.

test/functional/application_controller_test.rb

@@ -76,13 +76,12 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
     end
 
     context "when a user has an invalid username" do
-      should "show a warning banner" do
+      should "redirect to the name change page" do
         @user = create(:user)
         @user.update_columns(name: "foo__bar")
 
         get_auth posts_path, @user
-        assert_response :success
-        assert_select "#invalid-name-notice"
+        assert_redirected_to new_user_name_change_request_path
       end
     end