Bulk entity creation via API

[ktuite]

Closes getodk/central#573

Uses POSTing to existing /v1/projects/1/datasets/people/entities endpoint so it can handle

• single entities with just {uuid, label, data}
• multiple entities with {entities: [{}, {}], source: {name, size}}

For the source object,

• source is required
• name within source is required (not sure if it should be but it will help with displaying on the front end...)
• size (meant to represent file size)

I was thinking about how to handle the bulk SQL insert and decided it was easiest to think/build two separate queries, one for the entities table and another for the entity_defs table. Elsewhere the code, we have complex queries for inserting a single entity's data into both tables at once, but it didn't seem reasonable to try to do that with multiple entities.

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

Tests, trying it out.

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

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?

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

Documentation is included in this PR!!

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

[ktuite]

Interactive review with @matthew-white