Allow NULL values in the ``Application`` ownership.

https://github.com/madewithbytes/us_ignite/issues/152

Allowing nullable ownership by setting the User FK to ``NULL``
on removal.
Allow Null values in the ``Challenge`` ownership.

https://github.com/madewithbytes/us_ignite/issues/152

Allowing nullable ownership by setting the User FK to ``NULL``
on removal.
Allow NULL values in the ``Event`` ownership.

https://github.com/madewithbytes/us_ignite/issues/152

Allowing nullable ownership by setting the User FK to ``NULL``
on removal.
Set the ``guardian`` to ``NULL`` after user removal.

https://github.com/madewithbytes/us_ignite/issues/152

Allowing nullable ownership by setting the User FK to ``NULL``
on removal.
Set the ``author`` to ``NULL`` after user removal.

https://github.com/madewithbytes/us_ignite/issues/152

Allowing nullable ownership by setting the User FK to ``NULL``
on removal.
Allow NULL values in the ``Resources`` ownership.

https://github.com/madewithbytes/us_ignite/issues/152

Allowing nullable ownership by setting the User FK to ``NULL``
on removal.