Follow up to a509441148
This commit makes category and tag link in the sidebar consistent with the Everything link when the new New view experiment is enabled. In particular:
1. Category and tag links navigate to the per-category (or tag) `/new` view if there's at least one topic, and to `/latest` if there are no topics
2. Category and tag links only show the count of topics in `/new` without text
3. The Everything link navigates to the global `/new` view if there's at least one topic there, and to `/latest` if there are no topics in `/new`.
Internal topic: t/77234.
aad0d5fcfb