real-time update: adjust replyCount up/down

this also fixes the connecting lines in the Sk-style view

thanks @ShittyKopper for reporting the bug!

NOTE: at this point, the `isDeletedRef` boolean is pretty much
useless, because we're directly removing deleted notes from the
`replies` array and therefore from the DOM (we were just hiding them,
before); I'm intentionally not touching `isDeletedRef` to simplify
merges from upstream