On environments, only 2 scopes are valid: available and stopped. If an
invalid scope is passed via the query params, the controller attempted
to use it and would return a 500 error.

This behaviour is still useful for the JSON API, and so we only fetch
the environments if the format is JSON.

However, as the environments are fetched via API after the HTML is
returned, there's no need to query the environments when the format is
HTML, meaning no need to use the scope. This stops invalid query
params (like ?scope=foo) from returning a 500 error. To stop the
resultant API calls from failing, we can check to see if the requested
scope is valid. If it is not, default to the available scope.

Changelog: fixed