(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/test_summary.py

@@ -3,16 +3,15 @@ Test of Summary tables. This has many test cases, so to keep files smaller, it's
 files: test_summary.py and test_summary2.py.
 """
 
+import logging
 import actions
-import logger
 import summary
 import testutil
 import test_engine
+from test_engine import Table, Column, View, Section, Field
 from useractions import allowed_summary_change
 
-from test_engine import Table, Column, View, Section, Field
-
-log = logger.Logger(__name__, logger.INFO)
+log = logging.getLogger(__name__)
 
 
 class TestSummary(test_engine.EngineTestCase):