[schema][manual] Add schema invite_only flag for streams

(imported from commit 6234b67b3c72912bf40e39007bbc884d20774806)

zephyr/models.py

@@ -92,6 +92,7 @@ class MitUser(models.Model):
 class Stream(models.Model):
     name = models.CharField(max_length=30, db_index=True)
     realm = models.ForeignKey(Realm, db_index=True)
+    invite_only = models.NullBooleanField(default=False)
 
     def __repr__(self):
         return "<Stream: %s>" % (self.name,)