Fix #4383: Save and display deleted usernames for mods+.

Name changes for deleted users are already visible to mods, so the only
thing we need to do here is to generate a name change before the user is
deleted.

test/unit/user_deletion_test.rb

@@ -27,7 +27,7 @@ class UserDeletionTest < ActiveSupport::TestCase
 
   context "a valid user deletion" do
     setup do
-      @user = create(:user, email_address: build(:email_address))
+      @user = create(:user, name: "foo", email_address: build(:email_address))
       @deletion = UserDeletion.new(@user, "password")
     end
 
@@ -41,6 +41,15 @@ class UserDeletionTest < ActiveSupport::TestCase
       assert_equal("user_#{@user.id}", @user.reload.name)
     end
 
+    should "generate a user name change request" do
+      assert_difference("UserNameChangeRequest.count") do
+        @deletion.delete!
+      end
+
+      assert_equal("foo", UserNameChangeRequest.last.original_name)
+      assert_equal("user_#{@user.id}", UserNameChangeRequest.last.desired_name)
+    end
+
     should "reset the password" do
       @deletion.delete!
       assert_equal(false, @user.authenticate_password("password"))