Reimbursement updates 2

Fix for ReimbursementPerformer
- Fix reimbursement simulation when some payment methods cannot be reimbursed
- 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.

I have these as separate commits since they're separate issues but let me know if you'd prefer me to squash them all into a single commit.