gemify respondus_soap_endpoint

This turns the vendor/plugin into a rails engine, and establishes the
convention of canvas-dependent rails engines/gems going into
gems/plugins, and everything in that dir being auto-required and added
to canvas spec runs.

closes CNVS-14286

test plan: regression test on the respondus quiz builder client. no
behavior should change.

Change-Id: I51b548418bcb70af77e87c2c2d62cac27ed4ab44
Reviewed-on: https://gerrit.instructure.com/38394
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>