This new endpoint allows clients to easily purge steering’s application
state. It is primarily designed for use in integration tests as outlined
in aura#387.

The endpoint is available in debug mode so that developers can inspect
the schema. However, any request to it is denied unless the
OPERATING_MODE environment variable is set to "tests". This is a
precaution for administrators that use the debug mode to analyze
application issues in production.

Closes #228

This endpoint adds a quick way for clients to request arbitrary image
sizes based on the functionality available in the versatileimagefield
package.

If we have more advanced use cases we could either take a look at
integrating libvips [1] or delegate requests to a thumbor instance [2].

[1] https://github.com/libvips/pyvips
[2] https://github.com/thumbor/thumbor

Closes #203

This reverts commit ad526281.