When tabbing through a topics list like /latest, /unread, /new etc. the
Replies column is announced as `<replies count> button` by screen
readers and it's not clear that number means the topic has that number
of replies. This commit adds an `aria-label` so the Replies column to
make it clear what that number means. The current copy of the
`aria-label` is "This topic has <replies count> replies".
0d6bb64c0f