Add waiting room to create programming groups (#1919)

Co-authored-by: Sebastian Serth <Sebastian.Serth@hpi.de>

app/models/user.rb

@@ -25,6 +25,7 @@ class User < ApplicationRecord
   has_many :events
   has_many :events_synchronized_editor, class_name: 'Event::SynchronizedEditor'
   has_many :pair_programming_exercise_feedbacks
+  has_many :pair_programming_waiting_users
   has_one :codeharbor_link, dependent: :destroy
   accepts_nested_attributes_for :user_proxy_exercise_exercises