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/spec/fabricators/chat_fabricator.rb

@@ -154,3 +154,5 @@ Fabricator(:chat_thread, class_name: "Chat::Thread") do
 
   after_create { |thread| thread.original_message.update!(thread_id: thread.id) }
 end
+
+Fabricator(:user_chat_thread_membership, class_name: "Chat::UserChatThreadMembership") { user }