Simplify debts between two users when a new debt is added

[IsaacCheng9]

Summary

• Created a helper function, processNewDebts.
  • Removed unnecessary helper functions updateDebt and createDebt and integrated them into this new function.
• Simplified debts between users by checking for a reverse debt and updating it accordingly if it exists.
  • When a user adds a new debt (directly or by adding an expense), the records are simplified so that only one debt exists between the pair of users.
    • X owes Y £5 and Y owes X £50 becomes:
      • Y owes X £45
• Improved response messages by including specific info about who the debt is going from and to.
• Removed unnecessary try-catch loops.

Related Issue

Fixes issue #42.

Testing

Tested the changes manually using Postman API calls for adding debts and adding expenses.

[gkSideProjects]

Everything looks good!