Reimbursement updates

- [**removed** doesn't apply to 2.2] Fix Reimbursement to inherit from Spree::Base
- Change Reimbursement tax calculator to receive the reimbursement instead of just the array of return items
  - This assists some systems like Avatax which may want more context
- Fix reimbursement create form to only try to include accepted items

cherry-picked from d3e39abdd4394e0877e23350414b7c02a91a84f0 in spree master