Check for broken links to externally-hosted files

[moseshll]

I would approach this by writing a cron job that could send warning e-mail, but I don't know if that fits with the heliotrope ecosystem.

[sethaj]

We might want to approach this like Notre Dame does. They have tests like this living outside of any application in it's own space: https://github.com/ndlib/QA_tests

This could also relate to phantomjs tests in general in #846