Follow-up from "Create global inner-border-radius variable"

The following discussion from !152268 (merged) should be addressed:

• @thutterer started a discussion: (+1 comment)

  @annabeldunstone One totally non-blocking thought I had while reviewing:

  Shouldn't we (longterm, definitely follow-up material) replace all these $border-radius-default instances with $gl-border-radius-base from gitlab-ui? The former has a value of 4px and the later has 0.5 * px-to-rem(8px), so also 4px.