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 see that this is the same implementation as for e.g. views. Just wondering if there is a whitespace after `Model:` (`Model: Foo` vs `Model:Foo`).

Furthermore, shouldn't the model_name be passed as an argument to the translation so it works correctly on LTR and RTL (if that matters)?