Removed ``PRIVATE`` from the ``Applications`` status.

This status is not used anywhere in the application.
Add support to external ``Challenges``.

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

Challenges can be run externaly, the integration with
external chalenges is a link to an extenal URL.
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.
Explain the flow in the form of the challenge.
Removed unused template.

The detail of the entry has been moved to:
``challenges/includes/entry_block.html``.
Add missing include template with detail of the entry.
Show status of the challenge entry in the detail page.