Handle deleted products

[rolandgeider]

At the moment, all the import scripts (OFF and USDA) only import new products and we don't know if any were deleted. I'm not sure how often this might happen, but if we import OFFs daily deltas with all the changes that happened, this might become an issue if they delete duplicated products, products that don't match their quality standards, etc.

Things to consider:

• how to find out which ones were removed (first manually, and if this is not worth the work, simply ignore them)
• don't delete local ingredients if they are used in plans, logs, etc.
• communicate this to instances that sync from us, so we would probably move the DeletionLog to the core and use that