fix-canvasid: save target_external_options in db

[nmaekawa]

• django cmd to update target_external_options for image targets
• hxat was pulling the manifest dynamically every time it needed the canvas-id, which is used as source_id in searches to catchpy. When we moved to a different infrastructure and all the manifest/canvases urls changed, the previous annotations became unsearchable!
• this fix saves the canvas-id in the db so it becomes a stable id; and clear canvas-id in the database next time the image target is accessed, the canvas-id will be pulled dynamically and saved in the db. Reruns with instructor annotations have to update the canvas-id in each annotation!