Reimbursement email
Fixes #5185
Display return acceptance errors in Admin UI

So admins know what rule caused an item to be rejected or placed in the manual_intervention_required state.
Fixes #5186
Add preference for automatically restocking returned items
Fixes #5188
Don't allow adding products to order if it's already shipped.
Fix typo.
Credit Reimbursement Type
Fixes #5187
Remove new & edit Admin pages for reimbursement types

These are managed through code (e.g. migrations) rather than the Admin UI.
Leave an index page to display the current state.

Also:
  - Fix a migration. It was failing silently because of cached column information

Fixes #5191
Order by reverse created_at in Admin::UsersController

Similar to https://github.com/spree/spree/pull/5024 - but for the Admin::UsersController.

(Note that because we're looking at a single user, it probably doesn't matter much whether we use `completed_at` or `created_at`.)

Fixes #5181