Update ``challenge_entry`` description.
Update ``ActiveChallengesManager`` to avoid listing removed items.

The ``active`` manager will avoid showing removed challenges.
Add ``is_open`` method to ``Challenges``.

The method helps to determine when a challenge is published
and the start and end date make it available to participants.
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.