converts context/_inbox_items js_block into js_bundle

test plan:
* regression test discussion replies

Change-Id: I19cd28a64843008c64d336c8dff4c5a5048e302d
Reviewed-on: https://gerrit.instructure.com/29165
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>