extract handlebars_tasks gem

fixes CNVS-11180

test plan:
 * ensure the jst rake commands operate correctly
	 * jst:compile
	 * jst:ember

Change-Id: Ibe3109cf2708abf08783e06b180eba9299472ccb
Reviewed-on: https://gerrit.instructure.com/29920
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>