(core) Update logging in sandbox code, and log tracebacks as single log messages.

Summary:
- Replace logger module by the standard module 'logging'.
- When a log message from the sandbox includes newlines (e.g. for tracebacks),
  keep those lines together in the Node log message.

  Previously each line was a different message, making it difficult to view
  tracebacks, particularly in prod where each line becomes a separate message
  object.

- Fix assorted lint errors.

Test Plan: Added a test for the log-line splitting and escaping logic.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3956

sandbox/grist/useractions.py

@@ -2,6 +2,7 @@
 from collections import namedtuple, Counter, OrderedDict
 import re
 import json
+import logging
 import sys
 from contextlib import contextmanager
 
@@ -27,8 +28,7 @@ import treeview
 
 from table import get_validation_func_name
 
-import logger
-log = logger.Logger(__name__, logger.INFO)
+log = logging.getLogger(__name__)
 
 
 _current_module = sys.modules[__name__]