Figure what to do with background and sizing utilities

Follow up from #2882813-14: Import progress component and utility classes for background and sizing BS Bootstrap theme

From /bs_base/themes/bs_bootstrap/sass/base/util.scss

// These two files are included to support only progress
// Bootstrap component for now.
// @todo With moving assets to libraries (https://www.drupal.org/node/2890739)
// this should be refactored.
@import "bootstrap/scss/utilities/background";
@import "bootstrap/scss/utilities/sizing";