Revisit using percentages in font-size

We have some discrepancies across font-sizes due to using font-size: [percentage] in a few places:

$code-font-size: 90%;

p.time {
  color: $gl-gray-400;
  font-size: 90%;
  margin: 30px 3px 3px 2px;
}

.commit-sha,
.ref-name,
.pipeline-number {
  @extend .monospace;
  font-size: 95%;
}

This can cause some weird behavior, depending on your OS/browser/font (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23090#note_118142526)