bootstrapified the filter input field on gradebook2
bootstrapified the filter input field on gradebook2

fixes CNVS-4852

test steps:
- visit the gradebook2
- ensure that the filter field is rounded

Change-Id: I87d1b4aef4b06de84253d2e6c4e78c5ce664af17
Reviewed-on: https://gerrit.instructure.com/19089
Reviewed-by: Ryan Florence <2d8900489f91ac272fd9ce9d5789db6522668ca2@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>
Product-Review: Eric Berry <b4e131942074b4e4dafdcc14a63e85b7d8e18577@instructure.com>

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