converts context/media_comment js_block into js_bundle

test plan:
* regression test context media comment

Change-Id: Iaabb16295dd30c86b6b8d5ede60964e9a4480e95
Reviewed-on: https://gerrit.instructure.com/29163
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stanley Stuart <stanley@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>