Support custom actions

[OrangeDog]

Extending SingletonModelAdmin and adding actions currently has no effect, as the actions bar is on the object list page that solo has removed.

The widget should be added to the object view page instead, to allow custom actions to be accessed.

[raphodn]

Now available with #88

You can set SOLO_ADMIN_SKIP_OBJECT_LIST_PAGE to False in your settings, and the object list page will not be skipped in the admin. Therefore you'll be able to see the actions you define in your admin.