Add association for Comments and RequestForComments

For the submission and comments, I mainly decided to use a `has_one` association. Based on the database schema, multiple request for comments could be allowed (i.e., for each file or submission), but this won't happen practically (since we always create new submissions and files). Hence, the `has_one` association is representing our relationship better.

app/models/exercise.rb

@@ -33,6 +33,7 @@ class Exercise < ApplicationRecord
   has_many :internal_users, source: :contributor, source_type: 'InternalUser', through: :submissions
   has_many :programming_groups
   has_many :pair_programming_waiting_users
+  has_many :request_for_comments
 
   scope :with_submissions, -> { where('id IN (SELECT exercise_id FROM submissions)') }