Allow customer to review transaction and add initials when adding credit to account

[jarrighi]

The idea is to give the user an extra opportunity to verify they want to ADD credit.
It would be nice also to give them a slot to leave their initials, which could help with auditing changes later.

[scholachoi]

Done so far:

• refactored addCreditPage to validate amount and show error messages
• if amount is valid, "Continue" button goes to ReviewPage
• added ReviewPage where customer can review transaction and enter initials

To do:

• Edit barter event model to save customer initials
• "Confirm" button sends request and switches to ConfirmationPage