Upgrade Rails to version 5.2.1 and adopt code & specs where necessary

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

app/controllers/exercises_controller.rb

@@ -12,9 +12,9 @@ class ExercisesController < ApplicationController
   before_action :set_file_types, only: [:create, :edit, :new, :update]
   before_action :set_course_token, only: [:implement]
 
-  skip_before_filter :verify_authenticity_token, only: [:import_proforma_xml]
+  skip_before_action :verify_authenticity_token, only: [:import_proforma_xml]
   skip_after_action :verify_authorized, only: [:import_proforma_xml]
-  skip_after_action :verify_policy_scoped, only: [:import_proforma_xml]
+  skip_after_action :verify_policy_scoped, only: [:import_proforma_xml], raise: false
 
   def authorize!
     authorize(@exercise || @exercises)
@@ -77,7 +77,7 @@ class ExercisesController < ApplicationController
   def create
     @exercise = Exercise.new(exercise_params)
     collect_set_and_unset_exercise_tags
-    myparam = exercise_params
+    myparam = exercise_params.present? ? exercise_params : { }
     checked_exercise_tags = @exercise_tags.select { | et | myparam[:tag_ids].include? et.tag.id.to_s }
     removed_exercise_tags = @exercise_tags.reject { | et | myparam[:tag_ids].include? et.tag.id.to_s }
 
@@ -160,19 +160,21 @@ class ExercisesController < ApplicationController
   private :user_by_code_harbor_token
 
   def exercise_params
-    params[:exercise].permit(:description, :execution_environment_id, :file_id, :instructions, :public, :hide_file_tree, :allow_file_creation, :allow_auto_completion, :title, :expected_difficulty, files_attributes: file_attributes, :tag_ids => []).merge(user_id: current_user.id, user_type: current_user.class.name)
+    params[:exercise].permit(:description, :execution_environment_id, :file_id, :instructions, :public, :hide_file_tree, :allow_file_creation, :allow_auto_completion, :title, :expected_difficulty, files_attributes: file_attributes, :tag_ids => []).merge(user_id: current_user.id, user_type: current_user.class.name) if params[:exercise].present?
   end
   private :exercise_params
 
   def handle_file_uploads
-    exercise_params[:files_attributes].try(:each) do |index, file_attributes|
-      if file_attributes[:content].respond_to?(:read)
-        file_params = params[:exercise][:files_attributes][index]
-        if FileType.find_by(id: file_attributes[:file_type_id]).try(:binary?)
-          file_params[:content] = nil
-          file_params[:native_file] = file_attributes[:content]
-        else
-          file_params[:content] = file_attributes[:content].read
+    if exercise_params
+      exercise_params[:files_attributes].try(:each) do |index, file_attributes|
+        if file_attributes[:content].respond_to?(:read)
+          file_params = params[:exercise][:files_attributes][index]
+          if FileType.find_by(id: file_attributes[:file_type_id]).try(:binary?)
+            file_params[:content] = nil
+            file_params[:native_file] = file_attributes[:content]
+          else
+            file_params[:content] = file_attributes[:content].read
+          end
         end
       end
     end
@@ -364,7 +366,7 @@ class ExercisesController < ApplicationController
       query = "SELECT user_id, MAX(score) AS maximum_score, COUNT(id) AS runs
               FROM submissions WHERE exercise_id = #{@exercise.id} GROUP BY
               user_id;"
-      ActiveRecord::Base.connection.execute(query).each do |tuple|
+      ApplicationRecord.connection.execute(query).each do |tuple|
         user_statistics[tuple["user_id"].to_i] = tuple
       end
       render locals: {