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/models/lti_parameter.rb

@@ -1,2 +1,5 @@
 class LtiParameter < ActiveRecord::Base
-end
+  scope :lis_outcome_service_url?, -> {
+    where("lti_parameters.lti_parameters ? 'lis_outcome_service_url'")
+  }
+end