Added support for reStructured text in admindocs for model docstrings

https://code.djangoproject.com/ticket/5405

Refered to https://github.com/django/django/pull/760 for help
@elvard wrote code to support it, but it needed to be modified, in order to merge
@gkmngrgn added tests that also needed to be modified
It is not. I could add it the same way it's in view_detail.html, or maybe that doesn't make sense:
```
{% if meta.Context %}
<h3>{% trans 'Context:' %}</h3>
<p>{{ meta.Context }}</p>
{% endif %}
```