This commit introduces a few experimental changes to the New topics list and "Everything" link in the sidebar:
1. Make the New topics list include unread topics
2. Make the Everything section in the sidebar link to the New topics list (`/new`)
3. Remove "unread" or "new" text next to the count and keep the count
4. The count is a sum of new and unread topics counts
All of these of changes are behind an off-by-default feature flag. I've not written extensive tests for these changes because they're highly experimental.
Internal topic: t/77234.
a509441148