Check compatibility of easy_thumbnailer with our prod mediafiles setup

[jeriox]

[christianschaeffer]

We could formulate this more generically: find a way to store images with thumbnails of different sizes in ephios. Since ephios plugins should not have library dependencies on their own, this must be provided centrally.

The library I copied here from in my prior implementation is one of many that exists, and as I see it there is no clear best choice. It might even be easier to implement something simple directly rather than have a complex configurable library with more features than we need.