Add foreign keys to schema

[segiddins]

Without foreign keys, tools such as https://github.com/graphile/postgraphile are unable to infer the relationships between the tables.

If you're interested in this addition, I'd be happy to come up with a PR adding it!

[droher]

Unlikely to get to it myself, but absolutely, would be great to have if you wanted to take a shot at it. The only requirement from my end would be to avoid causing issues with the output images that don't have FK support.