home: Add rel=dns-prefetch to Gravatar and any Zulip avatar host.

This hints to the browser that it should start DNS lookups for the
host, since it is likely to be necessary.  It is a softer form than
`rel-preconnect`, which may be unnecessary in these cases, if the
client has the resources cached already.

zerver/views/home.py

@@ -241,6 +241,10 @@ def home_real(request: HttpRequest) -> HttpResponse:
             "page_params": page_params,
             "csp_nonce": csp_nonce,
             "color_scheme": user_permission_info.color_scheme,
+            "enable_gravatar": settings.ENABLE_GRAVATAR,
+            "s3_avatar_public_url_prefix": settings.S3_AVATAR_PUBLIC_URL_PREFIX
+            if settings.LOCAL_UPLOADS_DIR is None
+            else "",
         },
     )
     patch_cache_control(response, no_cache=True, no_store=True, must_revalidate=True)