Add ability to pre-render thumbnails at upload time

This introduces an option that allows one to define a list of
thumbnail sizes to be rendered by async jobs at upload time.

Change-Id: Ida193699738c79aca333fa78b8b536d14a410841
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/301
Merge "Add ability to pre-render thumbnails at upload time"