Unsellable item flag

This change adds a boolean flag to the item table for items that are unsellable on the TP.

The flag will be set next time the update-items-from-api tool is run.

All items flagged as unsellable will be filtered out of the all-items API (verify that this is what you want done.)

A new option has been added to the item listing page to show these unsellable items. By default they aren't shown.