email_mirror: Clean up EMAIL_GATEWAY_EXAMPLE setting.

This isn't something that a user can ever modify, so it doesn't belong
in DEFAULT_SETTINGS.  While we're at it, we align the appearance of
the email gateway in the docs with whether this setting in the docs
will be valid.

zerver/lib/integrations.py

@@ -133,7 +133,7 @@ class BotIntegration(Integration):
 class EmailIntegration(Integration):
     def is_enabled(self):
         # type: () -> bool
-        return settings.EMAIL_GATEWAY_BOT != ""
+        return settings.EMAIL_GATEWAY_PATTERN != ""
 
 class WebhookIntegration(Integration):
     DEFAULT_FUNCTION_PATH = 'zerver.webhooks.{name}.view.api_{name}_webhook'

zproject/settings.py

@@ -191,7 +191,6 @@ DEFAULT_SETTINGS.update({
     # These are undocumented, and we don't set them in dev_settings.py
     # or test_settings.py , either.
     # TODO: document them.
-    'EMAIL_GATEWAY_EXAMPLE': '',
     'EMAIL_GATEWAY_PASSWORD': None,
     'EMAIL_GATEWAY_EXTRA_PATTERN_HACK': None,
     'STAGING': False,
@@ -562,6 +561,8 @@ for bot in INTERNAL_BOTS:
 
 if EMAIL_GATEWAY_PATTERN != "":
     EMAIL_GATEWAY_EXAMPLE = EMAIL_GATEWAY_PATTERN % ("support+abcdefg",)
+else:
+    EMAIL_GATEWAY_EXAMPLE = ""
 
 DEPLOYMENT_ROLE_KEY = get_secret("deployment_role_key")