Request entity versions in EntityShow, not diffs

[matthew-white]

This change is a precursor to getodk/central#505 and getodk/central#507. On the entity detail page, we need additional information about entity versions (from the …/versions endpoint). Once we have that information, we will no longer need information from the …/diffs endpoint that the page currently requests. In other words, this PR replaces the request to …/diffs with a request to …/versions.

What has been done to verify that this works as intended?

Just making sure that tests continue to pass.

Why is this the best possible solution? Were any other approaches considered?

There's now a little more logic around the diff shown in EntityFeedEntry. For example, we can use serverDiff to determine the names of changed properties, but we also need to determine which version was the server version at the time in order to retrieve its value for each changed property. For that reason, I've moved the diff into its own component, EntityDiff. That component will get more complicated as we work on getodk/central#507.

Before submitting this PR, please make sure you have:

• run npm run test and npm run lint and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced