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/db/migrate/20230510142249_add_user_chat_thread_memberships.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class AddUserChatThreadMemberships < ActiveRecord::Migration[7.0]
+  def change
+    create_table :user_chat_thread_memberships do |t|
+      t.bigint :user_id, null: false
+      t.bigint :thread_id, null: false
+      t.bigint :last_read_message_id
+      t.integer :notification_level, default: 2, null: false # default to tracking
+      t.timestamps
+    end
+
+    add_index :user_chat_thread_memberships,
+              %i[user_id thread_id],
+              unique: true,
+              name: "user_chat_thread_unique_memberships"
+  end
+end