Add easy way to disable inventory tracking.

Currently inventory is always being tracked (resulting in backorders if not replenished).
This PR adds a simple NoopInventoryOperator and a configuration switch.
Add Customer Welcome Email
Optimize cart totals retrieving from menu builder
Nesting email settings & configuration
Issue #895 taxon selection optimization
Merge pull request #901 from umpirsky/fix/issue-895-taxon

Issue #895 taxon selection optimization
Fix VariantCombinationValidator and backend variants display to work with products without options.
Fix template condition to keep tests passing.
Update Payum requirements in PayumBundle
Removing address forms from backend user form
Removing outdated scenario step.
Add security step event
Add OrderUserListener to cart initialize event
Merge pull request #890 from Richtermeister/admin-create-customer

Removing address forms from backend user form
Merge pull request #900 from winzou/frontend/menu

Optimize cart totals retrieving from menu builder
Merge pull request #903 from Richtermeister/product-without-options

Fix variant validation and backend display for products without options.
Merge pull request #878 from Richtermeister/customer-welcome

Add Customer Welcome Email
Add EventListener to complete Order
Add post creation Order events
Set OrderShippingState to checkout by default