Features/entities/relevant to conflict flag

[sadiqkhoja]

Part of getodk/central#504

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

Added tests

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

The name of querystring parameter could be shorter but we came to consensus on relevantToConflict in Slack

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

None

Does this change require updates to the API documentation? If so, please update docs/api.md as part of this PR.

Yes

Before submitting this PR, please make sure you have:

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

[matthew-white]

I haven't looked at tests yet, but I thought I'd start with these comments. It's looking good so far!

[matthew-white]

I think there are a few places where we need more test coverage, but besides that, I only have small suggestions to make. ✅ Entity updates via submission are a complex feature, so I really think we'll benefit from having excellent test coverage.