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/send_email.py

@@ -8,16 +8,18 @@ from zerver.models import UserProfile, ScheduledEmail, get_user_profile_by_id, \
 
 import datetime
 from email.utils import parseaddr, formataddr
+import logging
 import ujson
 
 import os
 from typing import Any, Dict, Iterable, List, Mapping, Optional, Text
 
-from zerver.lib.logging_util import create_logger
+from zerver.lib.logging_util import log_to_file
 
 ## Logging setup ##
 
-logger = create_logger('zulip.send_email', settings.EMAIL_LOG_PATH)
+logger = logging.getLogger('zulip.send_email')
+log_to_file(logger, settings.EMAIL_LOG_PATH)
 
 class FromAddress:
     SUPPORT = parseaddr(settings.ZULIP_ADMINISTRATOR)[1]