Refund preferences & Reimbursement Items

- Allow admins and customers to specify refund types
  - Provide a field for customers to specify a refund method preference at RMA time
  - Allow admins to specify an override refund method at reimbursement time
- Break out reimbursement_items from return_items into a separate DB table
  - Paves the way for non-return reimbursements. It also makes ReturnItem a bit simpler.

Minor:

- Fix the x icon on select2 selectboxes.
- Convert CustomerReturnsController into a ResourceController

Generated by @jordan-brough and @richardnuno