Deleting a user doesn't delete their chats

We discovered that after deleting a user, their account will be empty as expected, but their chats will still be visible in AdminFE. Maybe we need to delete their chat message references also.

cc @mkfain

The user deletion right now goes through the activities with this query

  @spec by_author(query, User.t()) :: query
  def by_author(query \\ Activity, %User{ap_id: ap_id}) do
    from(a in query, where: a.actor == ^ap_id)
  end

But what we really need is a function that can do that and can look up all the ids referencing the user_id that is being deleted in the chats table, and then join on chat_message_references table to get the object_id so we know which objects can be deleted.

assigned to @minibikini

mentioned in merge request !3341 (closed)

added Check if still valid bug labels

unassigned @minibikini