Add social sharing features

[idreyn]

We want folks to be able to link to a given journey on social media, so we need a control that lets them do that — ideally using Twitter/FB/whatever share cards. Bonus points if we can dynamically generate images for a given journey and store them on S3 or something!

[danielreynoldsma]

I've been thinking about this, would it make sense to have some sort of link shortening system? Currently, the URLs are kind of long.