Follower requests: Utilize object index.

Closes #677

lib/pleroma/user.ex

@@ -613,9 +613,10 @@ def get_follow_requests_query(%User{} = user) do
         ),
       where:
         fragment(
-          "? @> ?",
+          "coalesce((?)->'object'->>'id', (?)->>'object') = ?",
           a.data,
-          ^%{"object" => user.ap_id}
+          a.data,
+          ^user.ap_id
         )
     )
   end