Updated disabled input text color

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

Current disabled input text color is #333

• Current Design: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-form-form-input--disabled

Sketch disabled input text color is #707070

• Sketch Design: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/forms-spec-previews

Changes

1. Updates text color of disabled inputs to #707070
2. Updates testing snapshots