Add absolute URL to data dict

We need to update the translucent_data Django template filter, so that we've access to the Django's object URL in ES6.

I would suggest that…

• we update the translucent_data template filter so that…
• in case a absolute_url property or get_absolute_url method is defined on the model, get the URL so that…
• we can add the URL as _url property to the data dict

In the c-table component, we then have to ignore / skip the rendering of all fields starting with _. This should also be doucmented somewhere, that there's meta data available, starting with an underscore _ (probably in the template filter).

We need that, because we want to release HACTAR (#64 (closed) & confirm/hactar/core#112) and links are currently not accessible in Translucent / JS.

@tzambellis can you have a look at it? this is urgent, as Stefan is still waiting for the new release.