Add view to withdraw an ``Entry`` from a ``Challenge``.

https://github.com/madewithbytes/us_ignite/issues/114

In order for users to edit an accepted or submitted
``Entry`` to a ``Challenge`` they need to withdraw it.

This is done to avoid a scenario where the entry
is updated after it has been moderated.

The view requires a POST request to set the ``Entry``
status to ``DRAFT``.
Add mechanics in the entry detail page to withdraw.

https://github.com/madewithbytes/us_ignite/issues/114

The mechanics to withdraw an entry are initiated from
the detail page.