Fixed #23488 -- Added get_username to AnonymousUser.

`django.contrib.auth.models.AnonymousUser` now implements `get_username()` as `User` does.

See https://code.djangoproject.com/ticket/23488.

Putting the `versionadded` block within the list of attributes and methods of `AnonymousUser` (just above the mention of `get_username`), did not feel right: it seemed to apply to all attributes and methods below. Moving `get_username()` at the end did not feel right either. This is why I put the `versionadded` block separately.