refs dashboard#219

refs dashboard#282

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 #178