Move contents of Mixins to class where it is used if it is only used once

Some mixins are only used once, in these cases, we should move the contents of the mixin to where it is used.

Here is a list of mixins that are only used one time:

Class Name,Relative Path
green-status-color,app/assets/stylesheets/page_bundles/ci_status.scss, app/assets/stylesheets/framework/mixins.scss
basic-list-stats,app/assets/stylesheets/page_bundles/projects.scss, app/assets/stylesheets/framework/mixins.scss
hierarchy-path-chevron,app/assets/stylesheets/page_bundles/work_items.scss
diff-match-line,app/assets/stylesheets/highlight/_white_base.scss
white-base,app/assets/stylesheets/highlight/_white_base.scss, app/assets/stylesheets/highlight/themes/white.scss
btn-middle,app/assets/stylesheets/framework/buttons.scss
btn-red,app/assets/stylesheets/framework/buttons.scss
btn-white,app/assets/stylesheets/framework/buttons.scss
set-invisible,app/assets/stylesheets/framework/dropdowns.scss
svg-size,app/assets/stylesheets/framework/images.scss
on-webkit-only,app/assets/stylesheets/framework/mixins.scss, app/assets/stylesheets/framework/typography.scss
side-panel-toggle,app/assets/stylesheets/framework/mixins.scss, app/assets/stylesheets/framework/sidebar.scss
spinner-deprecated,app/assets/stylesheets/framework/source_editor.scss, app/assets/stylesheets/framework/spinner.scss
spinner-color-deprecated,app/assets/stylesheets/framework/spinner.scss
spinner-size-deprecated,app/assets/stylesheets/framework/spinner.scss