Set oui field as unique

Co-authored-by: Johanna England <[email protected]>

python/nav/models/oui.py

@@ -7,7 +7,7 @@ class OUI(models.Model):
  """Defines an OUI and the name of the vendor the OUI belongs to"""
 
  vendor = VarcharField()
- oui = models.CharField(max_length=17)
+ oui = models.CharField(max_length=17, unique=True)
 
  def __str__(self):
  return self.oui