allow html5 fullscreen for the canvadocs iframe

Adds the "allowfullscreen" attribute to the iframe created for Canvadoc
documents, signalling that Canvadocs can open into fullscreen.

fixes CNVS-13650

test plan:
 * configure canvadocs, and view a file with it
 * verify that the canvadocs iframe has "allowfullscreen" as an
   attribute

Change-Id: I84b8b01796caa7291246dca6050ccdf13cca3813
Reviewed-on: https://gerrit.instructure.com/36503
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Zach Wily <zach@instructure.com>