refactor: simple explicit header nav link padding

[wesleyboar]

Overview

• Simplify header nav link padding styles.
• Make header nav link padding consistent.
  ^{(i.e. overwrite Bootstrap's rem, which causes different space per client)}

Related

• required by feat: core-styles v2 header Core-Portal#1068

Changes

• renamed mixin from header__nav-link--tall to header--not-expanded
• added absolute sizes for dropdown padding
• scoped two styles to .s-header.

Testing & UI

• Menu has zero change from before, especially .nav-link padding.
• Menu has greater padding block when expanded.

not expanded	is expanded