convert external_content/cancel js_block into js_bundle

test plan:
* regression test external content cancel

Change-Id: I95923740e6c6da6fb6d426f09224177947746c19
Reviewed-on: https://gerrit.instructure.com/29154
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>