converts js_block in gradebooks/_assignments into js_bundle

test plan:
* regression test gradebook assignments

Change-Id: I5ece9df1770acdc8adf901f8fbf5e8d5e60c5413
Reviewed-on: https://gerrit.instructure.com/29145
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>