Create Management Command to clean up Renditions

[ephes]

The would be to:

• be able to recalculate all renditions after changing the compression quality or sizes settings for images
• removed unused renditions

[ephes]

Hmm, this is much harder than expected. It seems to be difficult to write tests which use Wagtails image.get_renditions-method, because it led to strange race conditions. Or maybe it's because the Wagtail code is using threads here and we are testing on sqlite which is not thread safe? I don't know, but I now put all my logic into a renditions module that will still work, even if I replace Wagtail later on...