Fixes pytest-dev#565

This is based on conversations in issues pytest-dev#553, pytest-dev#554, and pytest-dev#284

- Introduces a django_user fixture, which is a user with no additional
  priveleges. Similar to admin_user.
- A user_client, based on django_user. Similar to admin_client.
- 3 new RequestFactory based fixtures, for unauthenticated,
  authenticated, and admin users:

  - rf_unauth: relies on AnonymousUser, similar to django docs
    example.
  - rf_admin: relies on admin_user fixture
  - rf_user: relies on new django_user fixture

  In addition, these 3 fixtures differ from rf in that they:

  - Mimic AuthenticationMiddleware by add the user attribute to
    the request object
  - Mimic SessionMiddleware by adding the 'session' attribute
    to the request object. This is an in-memory session store object
    from ``django.contrib.sessions.backends.base.SessionBase``.

This fills the gaps left over by not having normal and unauthenticated
user objects, clients, and request factories (when admin ones were
available), and also covers a common case likely present in Django
projects - anonymous (not logged in), authenticated, and admin users.

Fixes pytest-dev#565

This is based on conversations in issues pytest-dev#553, pytest-dev#554, and pytest-dev#284

- Introduces a django_user fixture, which is a user with no additional
  priveleges. Similar to admin_user.
- A user_client, based on django_user. Similar to admin_client.
- 3 new RequestFactory based fixtures, for unauthenticated,
  authenticated, and admin users:

  - rf_unauth: relies on AnonymousUser, similar to django docs
    example.
  - rf_admin: relies on admin_user fixture
  - rf_user: relies on new django_user fixture

  In addition, these 3 fixtures differ from rf in that they:

  - Mimic AuthenticationMiddleware by add the user attribute to
    the request object
  - Mimic SessionMiddleware by adding the 'session' attribute
    to the request object. This is an in-memory session store object
    from ``django.contrib.sessions.backends.base.SessionBase``.

This fills the gaps left over by not having normal and unauthenticated
user objects, clients, and request factories (when admin ones were
available), and also covers a common case likely present in Django
projects - anonymous (not logged in), authenticated, and admin users.

Fixes pytest-dev#565

This is based on conversations in issues pytest-dev#553, pytest-dev#554, and pytest-dev#284

- Introduces a django_user fixture, which is a user with no additional
  priveleges. Similar to admin_user.
- A user_client, based on django_user. Similar to admin_client.
- 3 new RequestFactory based fixtures, for unauthenticated,
  authenticated, and admin users:

  - rf_unauth: relies on AnonymousUser, similar to django docs
    example.
  - rf_admin: relies on admin_user fixture
  - rf_user: relies on new django_user fixture

  In addition, these 3 fixtures differ from rf in that they:

  - Mimic AuthenticationMiddleware by add the user attribute to
    the request object
  - Mimic SessionMiddleware by adding the 'session' attribute
    to the request object. This is an in-memory session store object
    from ``django.contrib.sessions.backends.base.SessionBase``.

This fills the gaps left over by not having normal and unauthenticated
user objects, clients, and request factories (when admin ones were
available), and also covers a common case likely present in Django
projects - anonymous (not logged in), authenticated, and admin users.

Fixes pytest-dev#565

This is based on conversations in issues pytest-dev#553, pytest-dev#554, and pytest-dev#284

- Introduces a django_user fixture, which is a user with no additional
  priveleges. Similar to admin_user.
- A user_client, based on django_user. Similar to admin_client.
- 3 new RequestFactory based fixtures, for unauthenticated,
  authenticated, and admin users:

  - rf_unauth: relies on AnonymousUser, similar to django docs
    example.
  - rf_admin: relies on admin_user fixture
  - rf_user: relies on new django_user fixture

  In addition, these 3 fixtures differ from rf in that they:

  - Mimic AuthenticationMiddleware by add the user attribute to
    the request object
  - Mimic SessionMiddleware by adding the 'session' attribute
    to the request object. This is an in-memory session store object
    from ``django.contrib.sessions.backends.base.SessionBase``.

This fills the gaps left over by not having normal and unauthenticated
user objects, clients, and request factories (when admin ones were
available), and also covers a common case likely present in Django
projects - anonymous (not logged in), authenticated, and admin users.

Also, Update fixtures.py with changes from @blueyed