fix N+1 in gradebook importer.
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 <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Reviewed-by: Shahbaz Javeed <6692d7be3fa215e5483a121d6fb314def6049877@instructure.com>
Tested-by: Jenkins
QA-Review: KC Naegle <9fbecd016338f9910821a8a5d5d0ae4fc9abfaa5@instructure.com>
Product-Review: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

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