Serve a generic robots.txt with Rack::Static

The smoke tests expect this to be there, so we'll provide it.
Ultimately the robots.txt needs to be dynamic so it looks like this...

  User-agent: *
  Disallow:
  Sitemap: http://www.businesslink.gov.uk/sitemap.xml

...but we'll get to that later.
Install Nokogiri for XML sitemap generation
Return an empty sitemap for /sitemap.xml requests
Return a valid, populated sitemap.xml
Include full URLs (vs paths) in sitemap.xml
Check that paths with query parameters are included in sitemap.xml
Serve robots.txt dynamically from the app
Include sitemap URL in robots.txt