Add flag to determine when Challenge entries are active.

https://github.com/madewithbytes/us_ignite/issues/126

The flag will determine when the ``Entries`` for a
``Challenge`` are public while the challenge is
active or only after it has been completed and moderated.
Improve admin section for the ``Challenges``.

Users can be selected and filtered using the ``raw_id_fields``,
and help text describing the role of the user.
Update entries view publishing permissions.

https://github.com/madewithbytes/us_ignite/issues/126

The visibility of the entries is determined by several
factors:
- When a challenge is marked as ``hide_entries`` the entries
are only published after the ``Challenge`` date is in the past.
- The ``Entries`` can only be shown after they have been
``SUBMITTED``.
- ``Owners`` of the ``Application`` can preview the submission.