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'
     });

server/views/account/beta.jade

@@ -0,0 +1,17 @@
+extends ../layout
+block content
+    .text-center
+      h2 Welcome to the freeCodeCamp beta.
+      h3 We have disabled new sign ups on beta.
+    .text-left.col-xs-12.col-sm-8.col-sm-offset-2.col-md-6.col-md-offset-3
+      p You can try our new beta challenges, but your progress <bold>will not</bold> be saved.&thinsp;
+        a(href='/map') Click here to go to the beta challenges
+          | .
+      p Or you can return to our stable challenges, where your progress <bold>will</bold> be saved.&thinsp;
+        a(href='https://www.freecodecamp.org') Click here to go to the stable challenges
+          | .
+      p If you want to learn more about this beta, read our&thinsp;
+        a(href='https://forum.freecodecamp.org/t/frequently-asked-questions-about-the-freecodecamp-beta/134331') Frequently Asked Questions
+          | .
+
+      p Happy coding!