Modified to ViewSets & Routers

*REMARK*
Tests are broken - need to correct url reverse

Modified views from generic view to a ViewSet
in auth, file_manager, and items
custom_auth, nested routes, items tests

Changes 'auth' package to 'custom_auth' due to duplication in names
(contrib.auth..).
In virtualenv, one should use 'python manage.py ...' to operate on the
venv.

Installed drf-extensions and added nested routers.
So now accessing item's images (and all likewise fields) is simply done
like this:
/items/1/images/

In addition - fixed all 'items' tests.
Commented 'file_manager' tests. No need for them now.