Show URL for external-run Challenges.
Show URL for external-run Challenges.

https://github.com/madewithbytes/us_ignite/issues/116

When the challenge is run ouside US Ignite the
``Challenge`` will link to the external resource.

us-ignite/us_ignite,us-ignite/us_ignite,us-ignite/us_ignite,us-ignite/us_ignite,us-ignite/us_ignite