Add timeout to base_activity.py

An instance of requests.get was missing a timeout; this commit adds a timeout of 15 as used in other places in this codebase which already have timeouts.

bookwyrm/activitypub/base_activity.py

@@ -423,6 +423,7 @@ def get_activitypub_data(url):
                 "Date": now,
                 "Signature": make_signature("get", sender, url, now),
             },
+            timeout=15
         )
     except requests.RequestException:
         raise ConnectorException()