Do not enforce TO number uniqueness.

We don't know for a fact that TO number uniqueness is a requirement at
the database level. For now, remove the unique constraint so that users
can enter redundant TOs.

atst/models/task_order.py

@@ -40,7 +40,7 @@ class TaskOrder(Base, mixins.TimestampsMixin):
 
     pdf_attachment_id = Column(ForeignKey("attachments.id"))
     _pdf = relationship("Attachment", foreign_keys=[pdf_attachment_id])
-    number = Column(String, unique=True)  # Task Order Number
+    number = Column(String)  # Task Order Number
     signer_dod_id = Column(String)
     signed_at = Column(DateTime)