signals: Avoid importing bugdown from early initialization.

This fixes `tools/build-release-tarballs` failing.

zerver/signals.py

@@ -8,10 +8,6 @@ from django.template import loader
 from django.utils import timezone
 from typing import Any, Dict, Optional
 
-from zerver.context_processors import common_context
-from zerver.models import UserProfile
-
-
 def get_device_browser(user_agent):
     # type: (str) -> Optional[str]
     user_agent = user_agent.lower()
@@ -54,6 +50,11 @@ def get_device_os(user_agent):
 def email_on_new_login(sender, user, request, **kwargs):
     # type: (Any, UserProfile, Any, Any) -> None
 
+    # We import here to minimize the dependencies of this module,
+    # since it runs as part of `manage.py` initialization
+    from zerver.context_processors import common_context
+    from zerver.models import UserProfile
+
     if not settings.SEND_LOGIN_EMAILS:
         return