Streamline the API

The API originally followed the pattern of Urban Airship’s public API
v3, which included a wrapper object in each response, which provides
room for status and extra metadata.  However, it makes the inputs &
outputs asymmetric. This change strips that out, just using the raw
entity representations.

The main benefit? We can now easily import/export via the API with
curl, or similar simple HTTP tools. Dashboards can be copied from one
instance of Tessera to another simply by piping one curl command to
another.