Add select_related support for Site.objects.get_current

[Kris Ciccarello]

Since we cannot extend Site, a common pattern is to use a OneToOneField to extend a Site (i.e. with a SiteDetail class). When performing request.site = get_current_site(request) in middleware, it would be nice to be able to efficiently load any extended Site objects onto request.site at that time.

Given this class:

`
from django.contrib.sites.models import Site

class SiteDetail(models.Model):

site = models.OneToOneField(

Site, on_delete=models.CASCADE, primary_key=True, related_name="detail"

)

`

The following does not work:

`
Site.objects.select_related("detail").get_current(request)

AttributeError: 'QuerySet' object has no attribute 'get_current'

`

We could add support for something like this:

`
Site.objects.get_current(request, select_related=detail)
`

Which would allow the user to pass through any custom Site extensions without having to re-write the code in the Site manager. Given the use of SITE_CACHE inside of SiteManager, it seems like making this addition to its API would be a good step.

Does this make sense, or would another approach be better? Happy to make a PR if this would be considered.