force allowfullscreen attribute onto review iframes

Mentioned this issue in PR https://github.com/hummingbird-me/hummingbird/pull/126

> Also, Summernote seems to only add the allowfullscreen tag to Vimeo, and Youku embeds -- and they don't seem to have any configuration to change it. We may want to amend the review_helper.rb file to force-add it?