environments_mixin.js

To support environment folders in the UI on the Environments List
page, the environments index route previously returned one environment
per folder, excluding those other than the latest deploy. However, the
environtments dropdown on the metrics dashboard requires that any
environment be selectable.

To accommodate both use cases, support an optional 'nested' parameter
in the index route to return either a flat, complete response or a
nested response based on the use case in question. The new default
response structure is the flat response.