Add support for custom billing invoice email address

Fixes #782

data/database.py

@@ -288,6 +288,7 @@ class User(BaseModel):
   last_invalid_login = DateTimeField(default=datetime.utcnow)
   removed_tag_expiration_s = IntegerField(default=1209600)  # Two weeks
   enabled = BooleanField(default=True)
+  invoice_email_address = CharField(null=True, index=True)
 
   def delete_instance(self, recursive=False, delete_nullable=False):
     # If we are deleting a robot account, only execute the subset of queries necessary.