Simplify the status endpoint and return status

If one of our deps isn't ok then we should be sending a 5xx status.

There was a lot of dupe between the two status checks so I've pulled it
out into a function.

The status of the admin app is now dependant on the status of its
dependencies.
Merge pull request #36 from alphagov/improve-status-endpoint

Simplify the status endpoint and return status