DEV: Add messages_count to ChatChannel table (#19295)

This commit adds the messages_count column for ChatChannel messages,
which is the number of not-deleted messages in the channel.

This is not updated every time a message is created or deleted in a
channel, so it should not be displayed in the UI.
It is updated eventually via Jobs::ChatPeriodicalUpdates, which
will have additional functions in future after being introduced
here.

Also update these counts for existing channels in a post migration.

plugins/chat/db/migrate/20221202032006_add_chat_message_count_to_chat_channels.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class AddChatMessageCountToChatChannels < ActiveRecord::Migration[7.0]
+  def change
+    add_column :chat_channels, :messages_count, :integer, null: false, default: 0
+    add_index :chat_channels, :messages_count
+  end
+end

plugins/chat/db/post_migrate/20221202043755_update_chat_channel_message_counts.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class UpdateChatChannelMessageCounts < ActiveRecord::Migration[7.0]
+  def up
+    DB.exec <<~SQL
+      UPDATE chat_channels channels
+      SET messages_count = subquery.messages_count
+      FROM (
+        SELECT COUNT(*) AS messages_count, chat_channel_id
+        FROM chat_messages
+        WHERE chat_messages.deleted_at IS NULL
+        GROUP BY chat_channel_id
+      ) subquery
+      WHERE channels.id = subquery.chat_channel_id
+      AND channels.deleted_at IS NULL
+      AND subquery.messages_count != channels.messages_count
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end