Default actor_from_request hook supporting ds_actor signed cookie

Refs #784, refs #699

datasette/actor_auth_cookie.py

@@ -0,0 +1,18 @@
+from datasette import hookimpl
+from itsdangerous import BadSignature
+from http.cookies import SimpleCookie
+
+
+@hookimpl
+def actor_from_request(datasette, request):
+    cookies = SimpleCookie()
+    cookies.load(
+        dict(request.scope.get("headers") or []).get(b"cookie", b"").decode("utf-8")
+    )
+    if "ds_actor" not in cookies:
+        return None
+    ds_actor = cookies["ds_actor"].value
+    try:
+        return datasette.unsign(ds_actor, "actor")
+    except BadSignature:
+        return None