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
Well, the lines below it no longer have %s in them, so yes - remove it.