Change Express route from use() to get()

Using the verb is more intuitive.
Fix basic authentication

With help from @lennym: https://github.com/alphagov/spotlight/pull/529#issuecomment-43990093

The use of middleware seemed intermittent at best, which isn't exactly
what you want for authentication.
Merge pull request #529 from alphagov/express-route-use-to-get

Change Express route from use() to get()