logging: Reduce create_logger to new log_to_file.

The name `create_logger` suggests something much bigger than what this
function actually does -- the logger doesn't any more or less exist
after the function is called than before.  Its one real function is to
send logs to a specific file.

So, pull out that logic to an appropriately-named function just for
it.  We already use `logging.getLogger` in a number of places to
simply get a logger by name, and the old `create_logger` callsites can
do the same.

zerver/lib/digest.py

@@ -2,6 +2,7 @@ from typing import Any, Callable, Dict, Iterable, List, Set, Tuple, Text
 
 from collections import defaultdict
 import datetime
+import logging
 import pytz
 
 from django.db.models import Q, QuerySet
@@ -17,9 +18,10 @@ from zerver.models import UserProfile, UserMessage, Recipient, Stream, \
     Realm
 from zerver.context_processors import common_context
 from zerver.lib.queue import queue_json_publish
-from zerver.lib.logging_util import create_logger
+from zerver.lib.logging_util import log_to_file
 
-logger = create_logger(__name__, settings.DIGEST_LOG_PATH)
+logger = logging.getLogger(__name__)
+log_to_file(logger, settings.DIGEST_LOG_PATH)
 
 VALID_DIGEST_DAY = 1  # Tuesdays
 DIGEST_CUTOFF = 5