Cache site config per-request to reduce DB queries from template render (#23791)

ARCHBOM-1139: We were seeing about 150 queries to the django_site table
and over 200 to memcached just to load the login page. A lot of these
are via mako template rendering, but there are other sources, and this
will hopefully nip it in the bud.

openedx/core/djangoapps/site_configuration/helpers.py

@@ -4,8 +4,10 @@ Helpers methods for site configuration.
 
 
 from django.conf import settings
+from openedx.core.lib.cache_utils import request_cached
 
 
+@request_cached("site_config")
 def get_current_site_configuration():
     """
     Return configuration for the current site.