[WIP] Twitter/Facebook sharing for /mybadges badges

This adds the basics for Twitter and Facebook sharing of an earned badge under /mybadges. It's hard to test locally, and hard to talk about without having something to show people, so getting this to a staging environment would be great.

Known issues: 
- [ ] share copy totally invented by me
- [ ] buttons unstyled
- [ ] badge must be made public manually for sharing to work