Stop spree_sample:load creating a duplicate record

Spree::ShippingCategory.create!(:name => "Default") is run in the following migration https://github.com/spree/spree/blob/master/core/db/migrate/20130830001033_add_shipping_category_to_shipping_methods_and_products.rb therefore a create command here would create a duplicate if a db:migrate had been run prior. shipping_categories.rb could be removed from samples completely but users of db:reset (that uses db:schema:load and loads the db from schema not migrations) would need it to be created here.

Fixes #5161
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.

Fixes #5160
Make return item eligibility validator more extensible. Add a validator for RMA presence
Fixes #5159
Fix load_user_roles. Fixes #5141
Mark inventory units as shipped in order importer.
Allow posting to ResourceController without parameters

When the associated model doesn't require any attributes.
Needed this for some returns & exchanges stuff.
Similar to 3be886f7de8a2f8515633c4d1d218140dc4373bc

Also, beef up ResourceController specs a bit.

Fixes #5158
Create a default calculator to determine a return item's default refund amount
Fixes #5162
Reload to make sure shipment.inventory_units have the latest status so that shipment.backordered? return correct value
Fixes #5163
Enhances ability of extensions to customize line items and the entire cart composition process.

Fixes #5087
Extract current order params to a function.

The parameters used to find and create orders is duplicated in 3 places.
Extracting them to a function reduces duplication, and will allow store
maintainers to place conditions on what makes an order unique. An
example would be store restrictions like spree-multi-domain, that
currently have to use an ugly module_eval.

Fixes #5164
Updates changelog for extension options.

Fixes #5167
Update Admin UI for multiple reimbursements per customer return

Tweaks the admin ui a bit so that multiple reimbursements per customer return are displayed
correctly.

We originally only expected a single reimbursement per customer return. But with expedited exchanges
there may be a reimbursement that happens immediately (the exchange shipment) for any exchange items
in the RMA and another reimbursement that happens later when the return items arrive -- when the
other non-exchange items are reimbursed for money.  This new display logic is more robust in any
case.
Fixes #5171
Add message in reimbursements admin when the order has previously received a refund
Fixes #5173
Reimbursement Engine
Fixes #5175