Add unique constraint and index for PO number field in DB

Purpose:

• In order to guarantee PO number uniqueness across all orders a unique constraint needs to be added for PO number filed in DB.
  Right now the uniqueness is enforced by the validation that takes place when Order is created/update, but since there is a gap between validation and actual update of the Order it is not reliable and sufficient.
• Also it's expected that PO number will often be used for searching the Order, so indexing this field should improve the performance.

Acceptance criteria:

• Unique constraint is added for PO number field
• Index is created for PO number field