Performance Optimizations:

1. Add index on testrun.submissionId
2. replace lti_parameters.size with lti_parameters.present? to prevent COUNT query

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

db/migrate/20180823135317_add_index_for_testrun_submission_id.rb

@@ -0,0 +1,5 @@
+class AddIndexForTestrunSubmissionId < ActiveRecord::Migration
+  def change
+    add_index :testruns, :submission_id
+  end
+end