Update challenge entry validation.
Update challenge entry validation.

The ``is_open`` method takes care to validate that the
Challenge is open for submissions, in the decorator, the
logic was not visible and it wasn't possible to determine
at glance what was the validation being performed.

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