Fixed invalid dashboard due balance amount

[disalechinmay]

Description

The due balance amount was previously considering only unpaid invoices.
It no considers partially paid invoices too.
Total due balance = (total of unpaid invoices) + [total of balance amount (invoice total - credit) on partially paid invoices]

Related Issues

#786

Steps to Test

1. Go to the application.
2. Create an invoice (example - $100).
3. Register a partial payment (example - $30; current due = $100 - $30 = $70) against the invoice you created in above step.
4. Validate the due balance shown on the dashboard (it should be $70 as long as there is no other invoice on the system).

Checklist

• I have tested these changes
• I have updated the relevant documentation
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the codebase
• My changes generate no new warnings or errors
• The title of my pull request is clear and descriptive

[salahlalami]

@disalechinmay Thank you