Check for disabled cookies upon showing the login page

Upon viewing the login page, check that cookies are supported in the browser, otherwise show an error message.

This replaces the previous notice that showed when loading the CAPTCHA without a session started. As sessions would seldom be started for logged out users, most users who saw this notice didn't actually need to take action. The new check applies for the entire login page, preemptively sets a cookie to check the presence for, and will only show the notice if that cookie doesn't persist.