Updated disabled input styles and examples

This will resolve part of issue (#649 (closed))

Changes:

1. Updates the border color of disabled inputs

Testing and examples were also missing related to disabled inputs. Additional changes:

1. Adds an example disabled input to form_group
2. Adds a disabled input story to form_group
3. Updates input testing snapshots

Current disabled input

The current disabled input border color does not match our Sketch design:

• Sketch Design: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/forms-spec-previews
• Current Design: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-form-form-input--disabled

Updated disabled input