Add challenge entry detail page.

https://github.com/madewithbytes/us_ignite/issues/115

The owner can access the detail page for a submission.
Add ``get_absolute_url`` for an Challenge entry.

Useful helper for reversing the URL of the entry.
Redirect non-draft entries to the entry detail page.

https://github.com/madewithbytes/us_ignite/issues/115

Once the entry has been marked as ``SUBMITTED`` or has been
``ACCEPTED`` the page shows the detail of the entry, unless withdrawn,
and the entry would require moderation again.