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
2-2 fix for customer returns Admin UI

"button_to" cannot accept the "params" argument in Rails 4.0.
See http://api.rubyonrails.org/v4.1.4/classes/ActionView/Helpers/UrlHelper.html#method-i-button_to
vs http://api.rubyonrails.org/v4.0.5/classes/ActionView/Helpers/UrlHelper.html#method-i-button_to