Show full title as tooltip if nav item title has ellipsis

Some nav items in the new sidebar of our Navigation redesign can have very long titles (like "Merge requests settings") and then show an ellipsis to truncate that title.

To not loose any information, we should show the full title as a tooltip, but only when they truncate. We have GlTruncate component to do that.

See !130840 (comment 1543502803) for the discussion that spawned this follow-up issue.