Importers: create standard API response entities

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We will implement a set of standard API responses for import endpoints. To avoid breaking changes, we only require new endpoints to return this data. We may update existing endpoints to include these response bodies, but only additively.

A new response:

{
  "foo": "bar"
}

An old response, retro-fitted:

{
  "old_foo": "old_bar",
  "v2_response": {
    "foo": "bar
  }
}

• Martin's comment
• Luke's comment
• Me asking Justin about non-public API usage