External model number (RTC-32735)

'External Model Number' is added as column in the 'Items in promotions' grid in item view and in the offer group items grid. New column appears when an item list or segmentation at item, variant or model level is added to an offer group. This is visible by default only on model and variant level. Column cannot be shown when tenant is using non-model items (system parameter). 

Fix: no duplicates of items (RTC-33160)

Fixed issue in ItemSelectionItemImport job, which caused duplicated item rows in Promotion database after adding non-model item to offer on variant level (then such duplicates were added to offers anytime when adding such SKU to offer on any level).
During release, existing duplicates are removed. Unique indexes added to prevent duplicated items in future.