Refactor authentication token for new study-group-based authorization

app/models/authentication_token.rb

@@ -6,11 +6,12 @@ class AuthenticationToken < ApplicationRecord
   include Creation
   belongs_to :study_group, optional: true
 
-  def self.generate!(user)
+  def self.generate!(user, study_group)
     create!(
       shared_secret: SecureRandom.hex(32),
       user: user,
-      expire_at: 7.days.from_now
+      expire_at: 7.days.from_now,
+      study_group: study_group
     )
   end
 end