Include "rich replies" fallback in threaded reply messages

Circles Android is currently not detecting threaded replies: circles-android#226 (closed)

This would not be an immediate problem if Circles iOS also included the "rich replies" fallback mechanism in its threaded replies. See https://spec.matrix.org/v1.9/client-server-api/#fallback-for-unthreaded-clients

The example from that portion of the Matrix spec is:

// within an event's content...
"m.relates_to": {
  // The m.thread relationship structure
  "rel_type": "m.thread",
  "event_id": "$root",

  // The rich reply structure
  "m.in_reply_to": {
    // The most recent message known to the client in the thread.
    // This should be something with a high chance of being rendered by the other client,
    // such as an `m.room.message` event.
    "event_id": "$target"
  },

  // A flag to denote that this is a thread with reply fallback
  "is_falling_back": true
}