Adds `allow-top-navigation` to iframe sandbox attributes
Adds `allow-top-navigation` to iframe sandbox attributes

Summary:
The LearnStorm articles embed Typeform forms which contain a link at the end to award your badge. However, while this worked a year ago, this no longer works with modern browsers, which will complain with the following error:

"The frame attempting navigation of the top-level window is sandboxed, but the flag of ‘allow-top-navigation’ or ‘allow-top-navigation-by-user-activation’ is not set."

To fix this for now, we'll add the `allow-top-navigation` attribute for iframes. We'll re-evaluate this over the next few days, and consider adding a per-iframe tag in Perseus, or finding some other and better solution for solving this problem.

Our original discussion about the bug on Slack: https://khanacademy.slack.com/archives/C5B4CHKDE/p1505170557000296

Our discussion in the Perseus Slack channel about the problem: https://khanacademy.slack.com/archives/C550Q5AMS/p1505176049000190

Test Plan:
* Open a browser and navigate to `http://localhost:8080/partner-content/learnstorm-mindset-activities-17/activity-two/get-started-activity-two/a/activity-2-supercharge-your-growth`
* Fill out the embedded Typeform form and hit Submit. You should see a celebratory message, and a button that says "Claim your badge". When you click the button, you should be redirected to your KA profile successfully.

Reviewers: emily, nickbreen, mattdunnrankin

Reviewed By: mattdunnrankin

Subscribers: tom, jared

Differential Revision: https://phabricator.khanacademy.org/D39308

learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus