Feature request: Add SITE_NAME setting

[Yo-Yo Ma]

Along with the subject, I'd also propose abstracting the purposes that contrib.sites.requests.RequestSite serves out of the sites app, since the purpose it serves is more of a general one whose usefulness spans admin and auth, as well as third-party apps. The use of code from sites when sites is not installed demonstrates this point. So does checking whether an app is installed from inside that same app (is it an app or is it a lib?). If app A doesn't explicitly require app B, app A should properly function when I delete app B from my filesystem.

Adding a SITE_NAME setting, which would be used by the sites app in the absence of "django.contrib.sites" from INSTALLED_APPS would perfectly suffice, as a quick and simple solution.