Add a healthcheck for the content api

[JakeChampion]

During the recent Heroku incident which meant some apps were offline, the image service EU was online and serving requests even though most of the responses were a 400 due to the content api being offline.

If we have a healthcheck for the conten api then when the content api in the eu is down, our fastly config will failover to the us app which should be working if the us content api is up