Add "report" task (for a post-deploy health check)

This adds a report task, which performs a post-deploy health check on the live interactive.

It currently checks for:

* 404s (including JS-initiated downloads)
* JavaScript errors
* console logs

If any of the above are found, the task fails and it prints details of the issue.

Grunt plugin: https://github.com/callumlocke/grunt-report