Revert ReimbursementItem model

This removes ReimbursementItem model and squishes it back into the ReturnItem model.
We split this model out of ReturnItem and we still think that's a good idea.  But we've run into some issues with it so we'll have to look at adding it back in later when we can work through all the details.
i was running into this on wombat as well, i wonder if we need to scope to not only the accepted ones, but also the non-reimbursed ones?