fix: Add key to pre-existing usergroup items

[enjeck]

To reproduce the problem:

• Create a table and add a usergroup column with "multiple" turned on
• Add a new row with multiple users and save
• Try to remove one of the users/groups. All of the users get cleared

This happens because there's no key associated with the item. So, we therefore add keys to the pre-existing usergroup items. With keys, only the clicked item is cleared.