ROIs: allow to retrieve (and auto-create) cropped ROI images
Classification editor: display ROIs when hovering ROI icon
Classification editor: make ROI preview optional
ROIs: allow auto creation of cropped images on ROI creation

By default this is switched off, but it can be activated by setting
the ROI_AUTO_CREATE_IMAGE variable to True in the settings. If
activated, an asynchronous Celery task is launched after the creation of
the ROI.
ROIs: delete cropped image files (if any) when ROI is deleted
Upgrade celery, django-celery and kombu modules
ROIs: use locks for the image creation celery tasks

Using locks makes sure that the creation of a cropped image for one
particular ROI isn't done multiple times at the same time. Locks are
acquired before image tasks are scheduled and released after the async
task is done. Therefore, multiple tasks for the same ROI won't get
scheduled.
Classification editor: make ROI context menus wider