Add ability to download data template for uploading entities

[matthew-white]

Part of getodk/central#589.

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

New tests, as well as trying out the button locally. I observed that a CSV file was downloaded with the expected content.

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

I thought about using URL.createObjectURL(), but a data URL seemed a little simpler.

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