DEV: Create UserChatThreadMembership table and model (#21481)

This will enable us to begin work on user tracking
state for a thread so we can show thread-specific
unreads and mentions indicators. In this case are following
the core notification_level paradigm rather than the solution
UserChatChannelMembership went with, and eventually we
will want to refactor the other table to match this as well.

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

plugins/chat/lib/chat/notification_levels_extension.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module Chat
+  module NotificationLevelsExtension
+    extend ActiveSupport::Concern
+
+    class_methods do
+      def chat_levels
+        @chat_levels ||= Enum.new(muted: 0, normal: 1, tracking: 2, watching: 3)
+      end
+    end
+  end
+end

plugins/chat/lib/chat/user_extension.rb

@@ -8,6 +8,9 @@ module Chat
       has_many :user_chat_channel_memberships,
                class_name: "Chat::UserChatChannelMembership",
                dependent: :destroy
+      has_many :user_chat_thread_memberships,
+               class_name: "Chat::UserChatThreadMembership",
+               dependent: :destroy
       has_many :chat_message_reactions, class_name: "Chat::MessageReaction", dependent: :destroy
       has_many :chat_mentions, class_name: "Chat::Mention"
     end