add the allowfullscreen to contentitem

fixes PLAT-1291

test-plan:
embed a video or something that supports fullscreen via content-item
attempt to make it fullscreen
it should allow you to go fullscreen

Change-Id: I2d9b3bd4a4682927526cecf90b7c4a8a6ca4f6f2
Reviewed-on: https://gerrit.instructure.com/68564
Tested-by: Jenkins
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>

public/javascripts/LtiThumbnailLauncher.js

@@ -17,6 +17,7 @@ define([
     var placement = JSON.parse(element.attr('target')),
       iframe = $("<iframe/>", {
         src: element.attr('href'),
+        allowfullscreen: '',
         width: placement.displayWidth || 500,
         height: placement.displayHeight || 500
       })