Provide a landing page

[iainlane]

So that you can type your filters and get redirected to the right URL with them urlencoded.

There's a start of one in static/index.html. It needs to get served somehow.

Ideas:

• Use Cloudfront
• Serve it ourselves directly

It should be cached so the Lambda doesn't get invoked every time.

As far as I know, since we have a custom domain, we have some kind of hidden Cloudfront distribution already already. So maybe we can do it ourselves with cache control headers, and save the extra layer.