Specs for ReimbursementPerformer

- Add specs for ReimbursementPerformer

cherry-picked from 63c7fbea993b9c3d0620cbcf9cf031ee66cd4615 in spree master
Reimbursement Admin UI updates

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

cherry-picked from 896c2a59628c571962fad06cc09320be3033d756 in spree master
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.

cherry-picked from 72356172c1209bce43ac07172953cc5aae0de803 in spree master