Redirect users to the Related Object when Switching Sites

 Fixes #159.
The `object_id` isn't validated as an integer/digits by the URL pattern and if you try `.get(pk='foo')` it will raise an exception. This catches that error beforehand. This is based on the default [admin implementation][0] of `get_object` which is sadly [broken by Mezzanine][1] because the `change_view` doesn't use `get_object`.

[0]: https://github.com/django/django/blob/1.6.5/django/contrib/admin/options.py#L540
[1]: https://github.com/stephenmcd/mezzanine/blob/master/mezzanine/pages/admin.py#L107