Align contextual navigation links design with the Design System.

Contextual navigation links are displayed at the top of some pages (Issue, Tag, Take Action, Evergreen, and Post pages). They are also used in the Articles block.

Tasks:

• add new states for each contextual navigation link (post type / category / tag) = focus / visited / visited hover.
• change the link & icons color for hover & onclick (active) states.
• Replace uppercase text by lowercase text for Post type links.