Add an index page with a simple form on it

This PR adds the simple functionality of showing an index page with a simple form on it from where you can generate screenshots.

I've refactored quite a bit of the original code. The following happened:
 * Switched from flatiron to express, as this seems to be better inline with other okfn apps
 * Created an `/api/generate` endpoint, which takes the same parameters as the old ones
 * Removed the `imagemagick` node module, as we can get away with just specifying the window size options in the `webshot` module.
 * Removed the `union` module as it was not used

You can run the app by running:
`node app`

and browsing to `http://localhost:5000`