converts content_imports/files js_block to js_bundle

test plan:
* regression test content import files

Change-Id: Ia7348b8c73d1d7528da1df546a91223d0916a2a0
Reviewed-on: https://gerrit.instructure.com/29166
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>