FEATURE: Chat header icon indicator preference (#20474)

This commit allows the user to set their preference vis-a-vis
the chat icon in the header of the page. There are three options:

- All New (default) - This maintains the existing behaviour where
  all new messages in the channel show a blue dot on the icon
- Direct Messages and Mentions - Only show the green dot on the
  icon when you are directly messaged or mentioned, the blue dot
  is never shown
- Never - Never show any dot on the chat icon, for those who
  want tractor-beam-laser-focus

plugins/chat/db/migrate/20230228062442_add_chat_header_indicator_preference.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddChatHeaderIndicatorPreference < ActiveRecord::Migration[7.0]
+  def change
+    add_column :user_options, :chat_header_indicator_preference, :integer, default: 0, null: false
+  end
+end