gemify acacemic_benchmark

closes CNVS-14265

move it from vendor/plugins to gems/plugins as a canvas-dependent
rails engine.

test plan: regerssion test on academic benchmark importing. no
behavior should change.

Change-Id: I2e06321c850cf6b981bbd3e313479b9d1f0bcb8e
Reviewed-on: https://gerrit.instructure.com/39852
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>