Suppressed the `if Site._meta.installed` pattern.

The purpose of this construct is to test if the django.contrib.sites
application is installed. But in Django 1.9 it will be forbidden to
import the Site model when the django.contrib.sites application isn't
installed.

No model besides Site used this pattern.

Refs #21719, #21923.
Avoided importing models from django.contrib.admin.

Fixed #21923. Refs #21719.