allow lti content to enter full screen mode

Test Plan
- lti content should be able to enter full screen mode

fixes PLAT-614

Change-Id: Icffa991ca618cc6aeab2ac5ba0743746ab8bac41
Reviewed-on: https://gerrit.instructure.com/40320
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>

Author: wbhumphrey

app/views/lti/_lti_message.html.erb

@@ -38,6 +38,7 @@
         </div>
     </div>
 </form>
-<%= iframe("about:blank", :name => 'tool_content', :id => 'tool_content', :width => '100%', :height => '400') %>
+<%= iframe("about:blank", :name => 'tool_content', :id => 'tool_content', :width => '100%',
+           :height => '400', allowfullscreen: "true", webkitallowfullscreen: "true", mozallowfullscreen: "true") %>
 
 <%= render @append_template if @append_template %>