make loading dropdown/list attribute schema idempotent

[ajtmccarty]

fixes #1631

given the short timeframe for this issue (1 day), I tried to take a simple approach and skip some of the refactoring I would like to do (I'd like to add a DiffBuilder class instead of the various quite large methods currently used in the Diff class)
I think my approach of deleting nodes from the Diff that do not actually include any changes is the most likely to work and least likely to break anything of the several other options I considered. Ideally, we wouldn't add nodes to the diff that do not include changes in the first place, but I did not think I had time to refactor Diff.get_nodes and I'm not sure what the test coverage looks like.

I added the JsonEqualityChecker class hoping that I could inject it into a new DiffBuilder class before I realized that I would not have time for that, but I decided to leave the class (instead of just using the are_json_equal function) in the hopes of building that DiffBuilder class some day.

also

• split up the core/utils.py file into a few smaller files inside of the new core/utils dir

• unit tests

[ajtmccarty]

issue fixed in #1659