ldap: Improve logging.

Our ldap integration is quite sensitive to misconfigurations, so more
logging is better than less to help debug those issues.
Despite the following docstring on ZulipLDAPException:

"Since this inherits from _LDAPUser.AuthenticationFailed, these will
be caught and logged at debug level inside django-auth-ldap's
authenticate()"

We weren't actually logging anything, because debug level messages were
ignored due to our general logging settings. It is however desirable to
log these errors, as they can prove useful in debugging configuration
problems. The django_auth_ldap logger can get fairly spammy on debug
level, so we delegate ldap logging to a separate file
/var/log/zulip/ldap.log to avoid spamming server.log too much.

zproject/settings.py

@@ -640,6 +640,7 @@ EMAIL_LOG_PATH = zulip_path("/var/log/zulip/send_email.log")
 EMAIL_MIRROR_LOG_PATH = zulip_path("/var/log/zulip/email_mirror.log")
 EMAIL_DELIVERER_LOG_PATH = zulip_path("/var/log/zulip/email-deliverer.log")
 EMAIL_CONTENT_LOG_PATH = zulip_path("/var/log/zulip/email_content.log")
+LDAP_LOG_PATH = zulip_path("/var/log/zulip/ldap.log")
 LDAP_SYNC_LOG_PATH = zulip_path("/var/log/zulip/sync_ldap_user_data.log")
 QUEUE_ERROR_DIR = zulip_path("/var/log/zulip/queue_error")
 DIGEST_LOG_PATH = zulip_path("/var/log/zulip/digest.log")
@@ -745,6 +746,12 @@ LOGGING = {
             'formatter': 'default',
             'filename': ERROR_FILE_LOG_PATH,
         },
+        'ldap_file': {
+            'level': 'DEBUG',
+            'class': 'logging.handlers.WatchedFileHandler',
+            'formatter': 'default',
+            'filename': LDAP_LOG_PATH,
+        },
     },
     'loggers': {
         # The Python logging module uses a hierarchy of logger names for config:
@@ -812,6 +819,11 @@ LOGGING = {
         # },
 
         # other libraries, alphabetized
+        'django_auth_ldap': {
+            'level': 'DEBUG',
+            'handlers': ['console', 'ldap_file', 'errors_file'],
+            'propagate': False,
+        },
         'pika.adapters': {
             # pika is super chatty on INFO.
             'level': 'WARNING',
@@ -854,6 +866,11 @@ LOGGING = {
         'zerver.management.commands.deliver_scheduled_messages': {
             'level': 'DEBUG',
         },
+        'zulip.ldap': {
+            'level': 'DEBUG',
+            'handlers': ['console', 'ldap_file', 'errors_file'],
+            'propagate': False,
+        },
         'zulip.management': {
             'handlers': ['file', 'errors_file'],
             'propagate': False,