add Poll#associated_shards
add Poll#associated_shards

This commit adds the Poll#associated_shards method so that
with_each_shard acts sane.

Test plan
- The poll_choices index endpoint should work correctly

Change-Id: Iabb0f3db25aec7e60ec1013424b534fa2c08102a
Reviewed-on: https://gerrit.instructure.com/36068
Reviewed-by: Derek DeVries <ff87a0776152510efd53bb1f4ea089dbbc90e730@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Matt Fairbourn <5f353e1c97bb9cc046b9db20beca0b9392f6f1b2@instructure.com>
Product-Review: Matt Fairbourn <5f353e1c97bb9cc046b9db20beca0b9392f6f1b2@instructure.com>

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