Refactor LTI parameters and add study group

* This change also requires that submissions in our test need to have a valid study group.

app/models/external_user.rb

@@ -2,6 +2,7 @@
 
 class ExternalUser < User
   validates :external_id, presence: true
+  has_many :lti_parameters, dependent: :destroy
 
   def displayname
     name.presence || "User #{id}"