Changed helper text font color

The helper text under input fields have a color of #919191

The Form Sketch (https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/forms-spec-previews/) styles the color as #707070

Changes

1. Updates the helper text font color
2. Overrides a bootstrap class, because the bootstrap class of text-muted is using !important. Additional details about this are mentioned in issue #646 (closed)
3. Update related snapshots for testing

Current helper text color:

Updated helper text color:

Closes #646 (closed)