Rename Replies to Comments

The following discussion from !165005 (merged) should be addressed:

• @slashmanov discussion: (+4 comments)

  Also, we now have a discrepancy between the used terms. We can collapse full threads or we can collapse replies to the threads. But the user always sees 'replies' mentioned. That means if we collapse a full thread with no comments the message will say: '1 reply', while in reality there are no replies in that thread.

  I think to avoid that confusion we should always use the term 'comments'.

  This also aligns with the global toggle for threads and thread comments.