Reimbursements model for Returns

- new Reimbursement model which encapsulates the concept of reimbursing a set of accepted return items, which will be reimbursed via one or more reimbursement methods (e.g., refunds, exchanges, store credits, etc)
- Ability to resolve "manual_intervention_required" returns items
- new Admin ReimbursementsController
- Provide preview of how reimbursements will be parceled out before performing them
- Add a default "ReimbursementPerformer" that figures out how to distribute a reimbursement
- ReturnItemsController for updating state
- extra logic from customer return into reimbursement
- fix suggested amounts things
- add generic "success/notice/error" state colors
- show page for customer returns
- remove customer return refund endpoint

TODO: TEST SETTING REIMBURSEMENT ATTRIBUTES