Global nav (9.0) UI improvement

Description

The global nav (introduced in 9.0) should be improved in HTML/CSS (UI) as a regression occurred in !12388 (merged) and fix for it spend for a long time in !12528 (merged).

Proposal

• Remove min-width/max-width as many as possible
• Use flexbox
• Apply text-overflow: ellipsis to limited selectors

Links / references

• !12528 (merged)
• !12388 (merged)

/label UX ~"technical debt"