Further decoupling of google_docs from models and controllers.

fixes: CNVS-12328

test plan:
  - as admin enable the google_doc plugin
  - as student visit `/profile/settings`
  - click 'Google Docs' under the 'Other Services' section
  - click 'Authorize Google Doc Access' in modal
  - when redirect to google, click 'Allow access'
  - verify success banner displays

Change-Id: I2b45ffa38ac0d1d810fb988335f4cd39154ae2a6
Reviewed-on: https://gerrit.instructure.com/32949
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>