Error toast when converting from rule builder to source code editor

An error pops up when converting a pipeline rule created in rule builder to source code.
Functionality is not impaired, but it is confusing.

## Expected Behavior

No error

## Current Behavior

## Possible Solution

Frontend needs to add `_scope` attribute to rule record

## Steps to Reproduce (for bugs)

1. Choose _Create pipeline rule_ on the _Manage rules_ tab
2. Build a valid rule
3. Click Convert to Source Code
4. Confirm by clicking _Create new Rule from Code_
5. Observe error popup

## Your Environment

* Graylog Version: 7.1