give verdantimages.Rendition an abstract base class too
eliminate references to the concrete Rendition model from AbstractImage, so that subclasses of AbstractImage can use their own AbstractRendition subclass
add get_image_model function to verdantimages.models, allowing installations to provide their own image class in lieu of verdantimages.Image
update core.rich_text module to recognise custom Image models
update TODO note on Document model in line with AbstractImage stuff
update verdantimages views to recognise custom Image models
Switch RCA app over to a custom image model
fix AbstractImage.search to search on the subclass in question, not necessarily Image