Make employment income manual try content store first

Try to serve this manual from the Content Store before serving the holding page.

We commonly use this manual as an example, and so do HMRC and their developers at iO1. This should also remove a small burden when it comes time for this manual to be published for real.

I wanted to make it return a 410 status instead of 200, but Slimmer currently rewrites that to look like a 5xx error. I've opened a [Pull Request](https://github.com/alphagov/slimmer/pull/97) which will mean Slimmer returns a generic 410 page, but it still wouldn't allow us to return our bespoke description of what's happened.