**What**
- Implement adding a line item to order (edit)
**How**
- _by implementing the following "flow"_
- generate a line item
- computing line item adjustments for that line item
- creating tax lines
- creating a change record
**Testing**
- **_integration tests_**
- check if line item and order item change objects are created (with correct tax lines)
- line item adjustments are generated if
- fixed discount is applied to cart
- percentage discount is applied
- **_unit tests_**
- ensure that methods from Inventory, LineItem, LineItemAdjustment etc. services are called
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RESOLVES CORE-495
884f36e8a8