Change 'Add Expense' dropdown to be more intuitive

[IsaacCheng9]

Specification

• Currently, adding an expense is a bit confusing for the user.
• We should change the behaviour under the three states:
  • When the page has initially loaded and hasn't started adding an expense
    • Show only one button, 'Add Expense', which opens the panel
  • When the user has started adding an expense
    • Show a button, 'Confirm Expense'
    • Also show a minimal-looking up arrow on the right side to minimise it (see arrow in VS Code terminal panel, shown below)
  • When the user has started adding an expense but minimised the panel
    • Show a disabled button (greyed out), 'Confirm Expense'
    • Also show the same arrow as in the previous scenario, but a down arrow instead of an up arrow

Screenshots