- Fix `/tasks.json` answering with a 500 since version 2.4.0, where the task files
  became `Task` objects that `jsonify` cannot serialize.
- Build the endpoint from the new `Task.as_dict()`, which writes the fields of the
  task config file plus the runtime data, i. e. whether a keepalive task is running.
- Leave the webhook tokens of a task out of that output.