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/controllers/user_exercise_feedbacks_controller.rb

@@ -41,9 +41,9 @@ class UserExerciseFeedbacksController < ApplicationController
     Sentry.set_extras(params: uef_params)
 
     @exercise = Exercise.find(uef_params[:exercise_id])
-    rfc = RequestForComment.unsolved.where(exercise_id: @exercise.id, user: current_user).first
+    rfc = RequestForComment.unsolved.where(exercise: @exercise, user: current_user).first
     submission = begin
-      current_contributor.submissions.where(exercise_id: @exercise.id).order('created_at DESC').first
+      current_contributor.submissions.where(exercise: @exercise).order(created_at: :desc).first
     rescue StandardError
       nil
     end