Add ``is_visible_by`` Organization method.

https://github.com/madewithbytes/us_ignite/issues/6

The visiblity determines whena user can preview an
application.
Autopopulate slug of the ``Organization`` from name.
Add ``Organization`` detail page.

https://github.com/madewithbytes/us_ignite/issues/6

The organizations have a profile that is listed
and available to preview to members.