panels: Show a banner for users with legacy desktop apps.

Users who are using ZulipDesktop or haven't managed to auto-update to
ZulipElectron should be strongly encouraged to upgrade.

We'll likely want to move to something even stricter that blocks
loading the app at all, but this is a good start.

zerver/views/compatibility.py

@@ -87,3 +87,17 @@ def check_global_compatibility(request: HttpRequest) -> HttpResponse:
                 and version_lt(user_agent['version'], android_min_app_version)):
             return json_error(legacy_compatibility_error_message)
     return json_success()
+
+def is_outdated_desktop_app(user_agent_str: str) -> bool:
+    user_agent = parse_user_agent(user_agent_str)
+    if user_agent['name'] == 'ZulipDesktop':
+        # The deprecated QT/webkit based desktop app, last updated in ~2016.
+        return True
+
+    if user_agent['name'] == 'ZulipElectron' and version_lt(user_agent['version'], '4.0.0'):
+        # Versions of the modern Electron-based Zulip desktop app with
+        # known security issues.  Versions before 2.3.82 won't
+        # auto-update; we may want a special notice to distinguish
+        # those from modern releases.
+        return True
+    return False