Refactor authentication token for new study-group-based authorization

spec/factories/authentication_token.rb

@@ -6,6 +6,13 @@ FactoryBot.define do
     shared_secret { SecureRandom.hex(32) }
     expire_at { 7.days.from_now }
 
+    after(:create) do |auth_token|
+      # Do not change anything if a study group was provided explicitly or user has no study groups
+      next if auth_token.study_group_id.present? || auth_token.user.study_groups.blank?
+
+      auth_token.update!(study_group_id: auth_token.user.study_groups.first.id)
+    end
+
     trait :invalid do
       expire_at { 8.days.ago }
     end