add ostatus subscribe to webfinger links

bookwyrm/views/wellknown.py

@@ -30,6 +30,10 @@ def webfinger(request):
                     "rel": "self",
                     "type": "application/activity+json",
                     "href": user.remote_id,
+                },
+                {
+                  "rel": "http://ostatus.org/schema/1.0/subscribe",
+                  "template": f"https://{DOMAIN}/ostatus_subscribe?acct={{uri}}"
                 }
             ],
         }