Room history: the user Id is displayed instead of their display name

[giomfo]

For unexpected reasons, the display name of a user becomes unknown as soon as he leaves the room. In consequence, the messages of this user are displayed in the room history with their user id. See the following screenshot:

To reproduce:

• UserA and UserB discuss in a room
• UserB leaves the room
• UserA triggers a clear cache to rebuild room histories
  -> messages from UserB are displayed with UserB Id instead of UserB display name in this room