Add comments to utilities.scss

Description

Add comments to utilities.scss to separate the styles into clear sections to match the styles in gitlab-ui.

For example:

// Text colors
.gl-text-blue-500 { @include gl-text-blue-500; }

// Background colors
.gl-bg-whatever { @include bg; }

Another example of a file in gitlab-ui:

https://gitlab.com/gitlab-org/gitlab-ui/blob/master/src/scss/utility-mixins/background.scss

This is a follow up from this discussion !20048 (comment 255249309).

Problem

Currently utilities.scss has no comments.

Suggested solution

Add comments to match gitlab-ui