Clean-up top navigation settings

[bartaz]

Done

• updates to latest Vanilla (and sass)
• aligns navigation breakpoint between juju.is and charmhub.io (Re-introduce 25/75 split in the top navigation charmhub.io#2067) - as charmhub has more items in nav (search and sign in)

QA

• Check demo
• Resize screen until navigation changes to mobile view - make sure it looks fine, without anything overflowing or wrapping
• Check the "All Canonical" navigation - it should change between desktop/mobile view at the same breakpoint as main navigation
• Check charmhub.io navigation from this PR and demo, make sure they are consistent

Issue / Card

Part of WD-18722

Screenshots

Top nav should still be visible when items fit:

All canonical global nav should be in mobile mode when top nav is:

[webteam-app]

Demo

Jenkins

demos.haus

[bartaz]

Needs more work, things change in navigation when you sign in to Charmhub and are affected by users name length.

[steverydz]

LGTM 👍