convert add_assignment js_block to js_bundle

test plan:
* regression on adding assignment

Change-Id: I34b0a7b9ec728bcda2c06aa2635f694c3c7d8d49
Reviewed-on: https://gerrit.instructure.com/29075
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>