Add ability to invalidate a sale

[gbenziv]

Implement the ability to invalidate a sale/transaction.

Sample use case:

• cashier unintentionally registered a transaction

There are two approach for this:

• Add an isVoid flag to the sale and/or saleItem entity.. OR
• Create a separate table for void transactions, and move the invalidated sale from Sales table to the new table.

Miscs:

• Add an option in accounting screen to select a transaction to invalidate
• Add an option in accounting screen to view all invalidated transactions