DEV: Add ChatThread model and DB table, and ChatMessage reference (#20106)

This new table will be used to automatically group replies
for messages into one place. In future additional functionality
will be built around the thread, like pinning messages, changing
the title, etc., the columns are just the main ones needed at first.
The columns are not prefixed with `chat_*` e.g. `chat_channel` since
this is redundant and just adds duplication everywhere, we want to
move away from this generally within chat.

plugins/chat/db/migrate/20230201012734_create_chat_threading_models.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class CreateChatThreadingModels < ActiveRecord::Migration[7.0]
+  def change
+    create_table :chat_threads do |t|
+      t.bigint :channel_id, null: false
+      t.bigint :original_message_id, null: false
+      t.bigint :original_message_user_id, null: false
+      t.integer :status, null: false, default: 0
+      t.string :title, null: true
+
+      t.timestamps
+    end
+
+    add_index :chat_threads, :channel_id
+    add_index :chat_threads, :original_message_id
+    add_index :chat_threads, :original_message_user_id
+    add_index :chat_threads, :status
+    add_index :chat_threads, %i[channel_id status]
+
+    add_column :chat_messages, :thread_id, :bigint, null: true
+    add_index :chat_messages, :thread_id
+  end
+end