run-dev.py: Fix incorrectly printed hostname for droplets.

Because the logic in print_listeners doesn't have access to computed
settings in dev_settings.py, we need to duplicate the special
IS_DEV_DROPLET logic for computing the default hostname.

There's still a secondary problem that this URL 404s.

tools/run-dev.py

@@ -366,7 +366,14 @@ def shutdown_handler(*args: Any, **kwargs: Any) -> None:
 
 
 def print_listeners() -> None:
-    external_host = os.getenv("EXTERNAL_HOST", f"localhost:{proxy_port}")
+    # Since we can't import settings from here, we duplicate some
+    # EXTERNAL_HOST logic from dev_settings.py.
+    IS_DEV_DROPLET = pwd.getpwuid(os.getuid()).pw_name == "zulipdev"
+    if IS_DEV_DROPLET:
+        default_hostname = os.uname()[1].lower()
+    else:
+        default_hostname = "localhost"
+    external_host = os.getenv("EXTERNAL_HOST", f"{default_hostname}:{proxy_port}")
     print(f"\nStarting Zulip on:\n\n\t{CYAN}http://{external_host}/{ENDC}\n\nInternal ports:")
     ports = [
         (proxy_port, "Development server proxy (connect here)"),

zproject/dev_settings.py

@@ -23,6 +23,7 @@ external_host_env = os.getenv("EXTERNAL_HOST")
 if external_host_env is None:
     if IS_DEV_DROPLET:
         # For our droplets, we use the hostname (eg github_username.zulipdev.org) by default.
+        # Note that this code is duplicated in run-dev.py.
         EXTERNAL_HOST = os.uname()[1].lower() + ":9991"
     else:
         # For local development environments, we use localhost by