Introduce JSONResponse.

[StevenXL]

This data type allows us to return a domain type in our handlers, even
if we eventually want to send JSON to the client.

See: https://tech.freckle.com/2015/12/21/servant-style-handlers-for-yesod/

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[StevenXL]

Tagging #1481 and #1494

[snoyberg]

Do you want to merge and release, or would you prefer I do?

[StevenXL]

Do you want to merge and release, or would you prefer I do?

If you could do so that would be great; I'll close the old PR and Issue.

[snoyberg]

Cool, doing so now, thanks!

[StevenXL]

Cool, doing so now, thanks!

Always a pleasure!