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/decorator.py

@@ -31,7 +31,7 @@ from io import BytesIO
 import urllib
 
 from typing import Union, Any, Callable, Sequence, Dict, Optional, TypeVar, Text, Tuple, cast
-from zerver.lib.logging_util import create_logger
+from zerver.lib.logging_util import log_to_file
 
 # This is a hack to ensure that RemoteZulipServer always exists even
 # if Zilencer isn't enabled.
@@ -45,8 +45,8 @@ else:
 ViewFuncT = TypeVar('ViewFuncT', bound=Callable[..., HttpResponse])
 ReturnT = TypeVar('ReturnT')
 
-## logger setup
-webhook_logger = create_logger("zulip.zerver.webhooks", settings.API_KEY_ONLY_WEBHOOK_LOG_PATH)
+webhook_logger = logging.getLogger("zulip.zerver.webhooks")
+log_to_file(webhook_logger, settings.API_KEY_ONLY_WEBHOOK_LOG_PATH)
 
 class _RespondAsynchronously:
     pass