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
I've looked into this, and to my knowledge, it seems it does not matter.