[IP-731]: Add another digit for quantity

[nielsdrost7]

This serves as an example. Please note, that you still have to alter the database table:

ALTER TABLE ip_invoice_items MODIFY COLUMN item_quantity decimal(10, 3);

First Rule of Programming: Don't run code from the internet when you don't understand it. It might break things.

Pull Request Checklist

• My code follows the code formatting guidelines.
• I have an issue ID for this pull request.
• I selected the corresponding branch.
• I have rebased my changes on top of the corresponding branch.

Issue Type (Please check one or more)

• Bugfix
• Improvement of an existing Feature
• New Feature