FIX: Add editing user ids to ChatMessage and ChatMessageRevision (#18877)

This commit adds last_editor_id to ChatMessage for parity with Post in
core, as well as adding user_id to the ChatMessageRevision record since
we need to know who is making edits and revisions to messages, in case
in future we want to allow more than just the current user to edit chat
messages. The backfill for data here simply uses the record's creating
user ID, but in future if we allow other people to edit the messages it
will use their ID.

plugins/chat/app/models/chat_message.rb

@@ -9,6 +9,7 @@ class ChatMessage < ActiveRecord::Base
   belongs_to :chat_channel
   belongs_to :user
   belongs_to :in_reply_to, class_name: "ChatMessage"
+  belongs_to :last_editor, class_name: "User"
   has_many :replies, class_name: "ChatMessage", foreign_key: "in_reply_to_id", dependent: :nullify
   has_many :revisions, class_name: "ChatMessageRevision", dependent: :destroy
   has_many :reactions, class_name: "ChatMessageReaction", dependent: :destroy
@@ -32,6 +33,8 @@ class ChatMessage < ActiveRecord::Base
 
   scope :created_before, ->(date) { where("chat_messages.created_at < ?", date) }
 
+  before_save { self.last_editor_id ||= self.user_id }
+
   def validate_message(has_uploads:)
     WatchedWordsValidator.new(attributes: [:message]).validate(self)
     Chat::DuplicateMessageValidator.new(self).validate
@@ -207,9 +210,11 @@ end
 #  message         :text
 #  cooked          :text
 #  cooked_version  :integer
+#  last_editor_id  :integer
 #
 # Indexes
 #
 #  idx_chat_messages_by_created_at_not_deleted            (created_at) WHERE (deleted_at IS NULL)
 #  index_chat_messages_on_chat_channel_id_and_created_at  (chat_channel_id,created_at)
+#  index_chat_messages_on_last_editor_id                  (last_editor_id)
 #