Added disabled input form_group example
Changes:
- Adds an example disabled input to form_group
- Adds a disabled input story to form_group
- Updates input testing snapshots
Reason:
The Sketch Design (https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/forms-spec-previews) shows the disabled input in a form_group and mentions the input field should explain why it is disabled.
Examples for the disabled input in a form_group are currently missing. The MR will add examples and a snapshot test to the system.
