Sharing

This implements basic sharing. It does not:

1. Provide a way to simply like the page. Do we want this? Should it be a separate button somehow? This seems like it is going to get cluttered quickly.
2. Claim to represent the final assets. Somebody who is better at assets should do this.
3. Solve the problem of supporting multiple environments for testing sharing. Don't think this is a problem but we need to remember to replace all the urls with the final pretty one before we deploy.

Note that I have not seen the meta properties working end to end because (as I understand it), FB/Twitter scrape these asynchronously. We will need to adjust these settings after we see it in action.