fix N+1 in gradebook importer.

An assignment's context was being loaded once per assignment per
student. This eliminates the N+1 by preloading the context with the
assignments.

closes CNVS-32664

test plan:
 - verify gradebook import still imports

Change-Id: I024b6022dc3dc4279ce3d168da9ae15489e6274f
Reviewed-on: https://gerrit.instructure.com/97031
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Reviewed-by: Derek Bender <djbender@instructure.com>
Reviewed-by: Shahbaz Javeed <sjaveed@instructure.com>
Tested-by: Jenkins
QA-Review: KC Naegle <knaegle@instructure.com>
Product-Review: Keith T. Garner <kgarner@instructure.com>

lib/gradebook_importer.rb

@@ -76,6 +76,7 @@ class GradebookImporter
   def parse!
     # preload a ton of data that presumably we'll be querying
     @all_assignments = @context.assignments
+      .preload(:context)
       .published
       .gradeable
       .select(ASSIGNMENT_PRELOADED_FIELDS)