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.
[TaxonomiesBundle] Cleanup models to match latest changes
Reduce Travis-CI build on main repository to only master branch
Translate address stuff
Translate personal home stuff
Translate order stuff
Translate general stuff, blocks and breadcrumbs
Translate exchange rates
Translate frontend stuff
Translate payments and promotions
Translate taxonomies
Translate user stuff
Translate menu and sidebar
Translate messages to German
Adapt and translate validators to German
Translate flashes to German
Translate messages to German
Translate validators to German
Ensure price and quantity are integer

For === comparisons
Don't delete Promotion when delete Order