Implement exchanges.

Fixes #5073
Specs for ReimbursementPerformer

- Add specs for ReimbursementPerformer
Reimbursement Admin UI updates

- Don't show edit button for reimbursement if reimbursement has been finalized
- Add an admin reimbursements show page
Fix rounding issue

Add some rounding in reimbursements to handle edge cases where multiple consecutive partial refunds
on the same order might result in rounding errors where we'd reimburse more than was originally
billed.