formatted_body isn't escaped when sending from Slack -> Matrix

[Half-Shot]

Leading to messages appearing broken:

{
  "content": {
    "body": "<3",
    "format": "org.matrix.custom.html",
    "formatted_body": "<3",
    "msgtype": "m.text"
  },
  "origin_server_ts": 1604687599444,
  "sender": "@slack_jobmachine_U049HHFSJ:jobmachine.org",
  "type": "m.room.message",
  "unsigned": {
    "age": 378585
  },
  "event_id": "$h_9beldUfb-IgLnn3qK0a9inFsl36yELSondXWzdtHs",
  "room_id": "!jPyDpGsiZKKZULwFzz:jobmachine.org"
}

[tadzik]

I cannot reproduce it anymore, even on develop

EDIT: Actually, <3 seems to just work now thanks to Element – the event source is the same. It's fixed properly in #781 though