feat(signup): Add disabled signup page (#15604)

Add a beta signup page and logic that will disable the signup page using
a environmental flag

server/boot/user.js

@@ -283,10 +283,16 @@ module.exports = function(app) {
     });
   }
 
+  const isSignUpDisabled = !!process.env.DISABLE_SIGNUP;
   function getEmailSignup(req, res) {
     if (req.user) {
       return res.redirect('/');
     }
+    if (isSignUpDisabled) {
+      return res.render('account/beta', {
+        title: 'New sign ups are disabled'
+      });
+    }
     return res.render('account/email-signup', {
       title: 'Sign up for freeCodeCamp using your Email Address'
     });