Rudder plugins » Change validation

This is part of the plan to "merge" both existing Elm apps in the change-validation settings page, as well as migrate the ChangeValidationSettings snippet to Elm, so that the change-validation settings page contains a single Elm app that renders the whole page.

As a first step, the 'Validate all' checkbox below the WorkflowUsers Elm app should be migrated so that it is part of the WorkflowUsers, as it is presently rendered by the ChangeValidationSettings snippet