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/submissions_controller.rb

@@ -115,7 +115,7 @@ class SubmissionsController < ApplicationController
     if @file.native_file?
       send_file(@file.native_file.path, disposition: 'inline')
     else
-      render(text: @file.content)
+      render(plain: @file.content)
     end
   end
 
@@ -140,7 +140,7 @@ class SubmissionsController < ApplicationController
       # probably add:
       # ensure
       #   #guarantee that the thread is releasing the DB connection after it is done
-      #   ActiveRecord::Base.connectionpool.releaseconnection
+      #   ApplicationRecord.connectionpool.releaseconnection
       # end
       Thread.new { EventMachine.run } unless EventMachine.reactor_running? && EventMachine.reactor_thread.alive?
 
@@ -329,7 +329,7 @@ class SubmissionsController < ApplicationController
 
   def set_file
     @file = @files.detect { |file| file.name_with_extension == params[:filename] }
-    render(nothing: true, status: 404) unless @file
+    head :not_found unless @file
   end
   private :set_file
 
@@ -362,7 +362,7 @@ class SubmissionsController < ApplicationController
     DockerClient.destroy_container(container)
   rescue Docker::Error::NotFoundError
   ensure
-    render(nothing: true)
+    head :ok
   end
 
   def store_error(stderr)