Updated helper-text font size
The sketch design displays the helper-text font-size of 14px
: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/forms-spec-previews/
The current helper-text appears to be smaller than 14px
: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-form-form-group--default
This merge request will update the helper-text font-size and line-height to match the Sketch design.