[backend] Make 'Mail Methods' link highlight as the active link when at mail_methods/edit

Fixes #2961
properly encapsulate Spree::Payment::Processing module

Fixes #3523
Reduced nested conditional to compound expression

Fixes #3527
Fetch deleted variant and product in line items

Fixes #3537
Remove update_attribute_without_callbacks in favour of update_column
Remove deprecated increment_quantity and decrement_quantity methods from LineItem
Add internal name for stock locations.

Includes light refactoring of stock_locations/index.html.erb from Ryan Bigg.

Fixes #3532
Add internal name for shipping_methods

Allows a place to store an internal name for a shipping method that helps associate the method to some backend context while still displaying a sensible name to shoppers.