Add query for visit visa applications

This should give us the number of unique visitors to a completion
page.

The regex should match:

    /customer/*/complete

But should NOT match:

    /customer/*/start/complete

So I've matched on everything except slashes.

https://www.pivotaltracker.com/story/show/71934810
Merge pull request #23 from alphagov/add-tier-2-visa-applications-count

Add query for visit visa applications