Enhance Django Admin integration

[cuducos]

The way I integrated the app in Django Admin is too hacky and can be improved in several ways:

• Use Django Admin action to re-start/pause jobs (instead of hardcoded buttons pointing to non-admin URLs)
• Customize the Django Admin add job (so we can remove the hardcoded form on the bottom of the screen)

We can minimize Django Admin template customization this way, and considerably enhance maintainability.