ISW - hide system rules in "Manage rules" tab

System rules with `_scope` = `GRAYLOG_SYSTEM_PIPELINE_RULE_SCOPE` cannot be modified or deleted like regular pipeline rules. They are automatically created / updated / deleted based on the corresponding input.
The only valid user action is to delete a system rule from a pipeline stage, when that routing is no longer desired.

There is no reason to open a system rule in the rules editor.

## Expected Behavior

System rules are not visible in the manage rules tab.
They are only visible in pipeline stages in the pipeline details view.

## Context

Don't clutter the UI with information or actions that don't make sense for the user.

## Your Environment

* Graylog Version: 6.2