Show non-kick leave messages

[hifi]

Is your feature request related to a problem? Please describe.

Non-kick leaves can also include a message. This is at least used by matrix-appservice-irc and Heisenbridge, likely others as well. It's especially important for IRC nick changes as it causes both a leave and join event and unless you see the reason you might not know the events are related.

Describe the solution you'd like

Show leave message if it exists.

Additional context

An example event in #heisenbridge room of an IRC user disconnecting:

{
  "content": {
    "membership": "leave",
    "reason": "Quit: Quit: Default quit message is difficult to set. If you don't see me returning, ask me? You will find me, if you want to",
    "displayname": "Aminda"
  },
  "origin_server_ts": 1629204787208,
  "sender": "@irc_libera.chat_aminda:heisenlab.vi.fi",
  "state_key": "@irc_libera.chat_aminda:heisenlab.vi.fi",
  "type": "m.room.member",
  "unsigned": {
    "replaces_state": "$EuiwNWlm5691mmBqV4bg8HMLX2fFdgaYJ3KdtEnN7D0",
    "prev_content": {
      "displayname": "Aminda",
      "membership": "join"
    },
    "prev_sender": "@irc_libera.chat_aminda:heisenlab.vi.fi",
    "age": 1272
  },
  "event_id": "$YM-12qwXboRaWD2bBLf9xgpAsMAXYmQU99nAAdY77-s",
  "room_id": "!NeZzxlfCZAOPeyCvLN:vi.fi"
}