This allows us to block use of the desktop app with insecure versions
(we simply fail to load the Zulip webapp at all, instead rendering an
error page).
For now we block only versions that are known to be both insecure and
not auto-updating, but we can easily adjust these parameters in the
future.
This is a prep-commit for the new navbar style, seperating
navbar_alerts.html from navbar.html in order to make the structure and
styles of navbar.html easier to tweak.
These templates have very little to do with each other to begin with
appart from the fact that they are both rendered at the top of the app.