Add manager method to match entries and apps.

The method returns an ``AppEntry`` named tuple that
matches ``applications`` and ``entries`` when available.
Add navigation link for the list of Challenges.

The challenges are ordered by the creation date.
Add detail page for a challenge.

https://github.com/madewithbytes/us_ignite/issues/113

Users should be able to see a challenge detail page when
the challenge is published.

Registered users will see a list of owned applications
to allow them to participate in the Challenge when it
is open.

The applications and their entries are matched in a
manager and bundled in a ``AppEntry`` named tuple.
Add ``get_absolute_url`` method to the ``Challenge`` model.
Add link to the challenge detail page from the challenge listing.