Table component: Extract styles from GitLab into gitlab-ui table

• Disable the pointer for non-sortable columns, since otherwise it's confusing that you can click on the column and it won't sort.

$b-table-sort-icon-bg-width: 1em;
$b-table-sort-icon-bg-height: 1em;

$b-table-sort-icon-bg-descending: url($gl-icon-arrow-down);
$b-table-sort-icon-bg-ascending: url($gl-icon-arrow-up);
$b-table-sort-icon-bg-not-sorted: url($gl-icon-arrow-not-sorted);

.gl-table {
  [aria-sort='none'] {
    &:not(:hover) {
      background-image: none !important;
    }
  }
}