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/user.rb

@@ -8,6 +8,7 @@ class User < ApplicationRecord
   belongs_to :consumer
   has_many :anomaly_notifications, as: :contributor, dependent: :destroy
   has_many :authentication_token, dependent: :destroy
+  has_many :comments, as: :user
   has_many :study_group_memberships, as: :user
   has_many :study_groups, through: :study_group_memberships, as: :user
   has_many :programming_group_memberships, as: :user
@@ -21,6 +22,7 @@ class User < ApplicationRecord
   has_many :testruns, as: :user
   has_many :interventions, through: :user_exercise_interventions
   has_many :remote_evaluation_mappings, as: :user
+  has_many :request_for_comments, as: :user
   has_many :runners, as: :contributor
   has_many :events
   has_many :events_synchronized_editor, class_name: 'Event::SynchronizedEditor'