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/home.py

@@ -24,6 +24,7 @@ from zerver.lib.streams import access_stream_by_name
 from zerver.lib.subdomains import get_subdomain
 from zerver.lib.users import compute_show_invites_and_add_streams
 from zerver.lib.utils import statsd, generate_random_token
+from zerver.views.compatibility import is_outdated_desktop_app
 from two_factor.utils import default_device
 
 import calendar
@@ -227,6 +228,7 @@ def home_real(request: HttpRequest) -> HttpResponse:
         debug_mode            = settings.DEBUG,
         test_suite            = settings.TEST_SUITE,
         poll_timeout          = settings.POLL_TIMEOUT,
+        insecure_desktop_app  = is_outdated_desktop_app(request.META["HTTP_USER_AGENT"]),
         login_page            = settings.HOME_NOT_LOGGED_IN,
         root_domain_uri       = settings.ROOT_DOMAIN_URI,
         max_file_upload_size  = settings.MAX_FILE_UPLOAD_SIZE,