Track balances between users based on expenses

[IsaacCheng9]

Specification

• We must track the amount of money owed from one user to every other user.
• To start with, we'll track this in a one-to-one relationship, with each document having the format:
  • from: user1
  • to: user2
  • amount: xyz (£)
• When adding an expense, the API will update an existing document if it already exists. Otherwise, it will create a new document as necessary (see below).
• This will be used for the list of group members, as it enables calculations for how much each member owes to one another.
  • For now, adding an expense will only ever increase a debt.
  • Debts will only be subtracted when a user pays off what they owe.