Make Smart Answers easier to run on its own

This pull request makes two changes which aim to make it easier to run the app on its own.

I've bumped Plek to 1.7.0, which includes changes that let a user override the location of a particular service (eg. static) by using an environment variable that's common across all apps. I've changed the configuration of `config.slimmer.asset_host` so that it requests the static host using Plek in the development environment.

I've also made a change to stop Smart Answers being dependent on the Content API in order to operate. The Content API dependency is not critical to the running of the app as it only supplies additional metadata such as the breadcrumbs and related links, and we already fail quietly if the Content API is running but an artefact doesn't exist.