Facebook share modal overlay

![Screen Shot 2013-02-15 at 7 50 35 AM](https://f.cloud.github.com/assets/1424627/160611/7a606f50-776e-11e2-87b6-4c7122e7f876.png)

If the user checks this checkbox, the FB share button will no longer display when you’ve pushed a badge to your backpack, and on the settings page the auto-share option will be set to “ON”.

If user selects auto-share, affects this page: https://github.com/mozilla/openbadges/issues/549