Adding missing singular/plural names for models in core

[Fixes #3674]
Cache result of insufficient_stock_lines so that it is not called for every single line item rendered on the cart page
Delete all inventory units (rather than destroy)

When a shipment was previously destroyed, it would call destroy on each InventoryUnit object. This took a long time if there were a lot of inventory units. There's no need to call destroy, as there's no callbacks in this model for destroy. All shipment needs to do is *DELETE ALL* its inventory units, and so this is what's going to happen now.
Only call update_order + update_inventory callbacks for LineItem if is destroyed or changed
Delete all shipping adjustments immediately prior to deleting all shipments in Order#create_proposed_shipments

What would happen before is that each shipment would find its adjustments and call destroy on them. We don't necessarily care about all that happening because once the new adjustments have been created then the order will be updated correctly.
When a shipment is destroyed, delete all shipping rates in one query, rather than 1 query per shipping rate
Check if a stock location has any of a variant in stock by using an exists? query, which is faster than Ruby
Handle 404 responses from alerts.spreecommerce.com

Fixes #3716
Fixing backend sub-menu navigation selection by making match_path for tabs work. Tests included now.

Fixes #3683
[backend] Remove conflict from CHANGELOG
no longer autocomplete values in product properties fields

Fixes #3695
remove horizontalNav js from admin this extension was causing warnings on the js consoles since it was using outdated jquery functions, and hasn't been updated this js plugin was here to allow ie7 < users to see the backend navigation using full-width

Fixes #3720
add spec to proof issue with variant update without product permalink.
fix the ActiveRecord::ReadOnlyRecord exception when the @product is null when update the variant

Fixes #3718
Fix syntax error.
Fix ReadOnlyError when destroying StockItem

StockMovement is a readonly object so we can't delete them. And by that
logic I believe we shouldn't clear stock items from the database either
otherwise we will have stock movements associated with non existing
stock items

Fixes #3727
Fixes #3723
Revert "fix the ActiveRecord::ReadOnlyRecord exception when the @product is null when update the variant"

This reverts commit 8660b46a546418d91ffec02124303e13b1a7a6a8.
Revert "add spec to proof issue with variant update without product permalink."

This reverts commit f39a2f6a8c9435595245c47b9b2fb86a00db9f31.
Prep for 2.0.5 release