Convert NewItems plugin to service helper

[sambhavp96]

No description provided.

[demiankatz]

@sambhavp96, as you may have already noticed, there appears to be an existing unit test for the NewItems helper -- hopefully you can rename and slightly adjust this to make things pass. I'll give this a closer review once that is done; let me know if I can assist with anything else in the meantime.

[demiankatz]

Thanks, @sambhavp96 -- see below for a few more suggestions!

[demiankatz]

Thanks for all the progress, @sambhavp96, this looks increasingly good! There are a couple of significant issues with the factory, though. Is the integration test suite passing for you? If so, we may have some gaps that need addressing there, since I would expect the code to be seriously broken by this -- but if you simply haven't had a chance to run the Mink tests yet, I understand! :-)

[demiankatz]

Thanks, @sambhavp96! During final testing of this, I discovered there was a gap in our automated test coverage of new items, so I opened #4947 to address it. This new test revealed some problems related to types, so I made some minor tweaks here to fix them. I also opened #4949 to correct some problematic annotations in existing code.

I am running the full test suite here one more time. Once it passes, I will merge this PR.