Preserve reply fallback on edit

[bwindels]

Fixes element-hq/element-web#10220

Example of an edited reply with these changes:

{
  "content": {
    "body": "> <@bruno4:matrix.org> doo\n\n * parrrrrr",
    "format": "org.matrix.custom.html",
    "formatted_body": "<mx-reply><blockquote><a href=\"https://matrix.to/#/!RbjQlmGUZlYnyhGpoZ:matrix.org/$1562590340825854VjKGO:matrix.org?via=matrix.org\">In reply to</a> <a href=\"https://matrix.to/#/@bruno4:matrix.org\">@bruno4:matrix.org</a><br>doo</blockquote></mx-reply> * parrrrrr",
    "m.new_content": {
      "body": "> <@bruno4:matrix.org> doo\n\nparrrrrr",
      "format": "org.matrix.custom.html",
      "formatted_body": "<mx-reply><blockquote><a href=\"https://matrix.to/#/!RbjQlmGUZlYnyhGpoZ:matrix.org/$1562590340825854VjKGO:matrix.org?via=matrix.org\">In reply to</a> <a href=\"https://matrix.to/#/@bruno4:matrix.org\">@bruno4:matrix.org</a><br>doo</blockquote></mx-reply>parrrrrr",
      "msgtype": "m.text"
    },
    "m.relates_to": {
      "event_id": "$1562591229829192bgNMu:matrix.org",
      "rel_type": "m.replace"
    },
    "msgtype": "m.text"
  },
  "event_id": "$1562591236829226iyhEm:matrix.org",
  "origin_server_ts": 1562591236665,
  "room_id": "!RbjQlmGUZlYnyhGpoZ:matrix.org",
  "sender": "@bruno4:matrix.org",
  "type": "m.room.message",
  "unsigned": {
    "age": 1702880
  },
  "user_id": "@bruno4:matrix.org",
  "age": 1702880
}

[jryans]

Thanks, looks good overall! 😁