[16.0] [FIX] sale_order_line_price_history: Add discount field only if discount group is enabled

[Shide]

If you try to add historic prices with discounts group deactivated, an error raises because discount field is not present on the view (kanban and tree)
This adds a check to see if discount is present on the record fields and add the key-value of discount if exists.

Added discount with the group to the kanban view (when defining fields)

Also added a known issue:
If you add the historic price from the kanban view, the sale order save button doesn't work. You need to go into the line and click save.

MT-8826 @moduon @CarlosRoca13 @yajo please review if you want :)