Move EE mini pipeline styles to page bundle CSS

This change removes global styles for the EE mini pipelines
to a page bundle file that's imported for EE mini pipeline
visible in MR pages.

ee/app/assets/stylesheets/pages/pipelines.scss → ee/app/assets/stylesheets/page_bundles/pipelines.scss

-@import './page_bundles/pipeline_mixins';
+@import '../../../../../app/assets/stylesheets/page_bundles/pipelines';
 
 .linked-pipeline-mini-list {
   &.is-downstream {