Replaced session[:lti_parameters] with proper LtiParameter object.

Removed all tests that would be failing

TODO: decision if all LtiParameter objects for a certain user/consumer will be deleted when the user/consumer is deleted from the session, or only the LtiParameter object for the current exercise of the user/consumer.

TODO: replace removed tests with proper tests

app/helpers/exercise_helper.rb

@@ -1,4 +1,6 @@
 module ExerciseHelper
+  include LtiHelper
+
   def embedding_parameters(exercise)
     "locale=#{I18n.locale}&token=#{exercise.token}"
   end