add allowfullscreen tags to the rce

test plan:
- add add a video in an iframe in the rich content editor
- it should be able to become full screen

fixes PLAT-647

Change-Id: I05ad81fdebc32353af9746605e9847947e614582
Reviewed-on: https://gerrit.instructure.com/41728
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>