Make IS_GITLAB_EE consistent

Currently:
1. If `IS_GITLAB_EE=` is used, it fails to generate Webpack
   as it cannot be parsed as JSON,
2. If `IS_GITLAB_EE=1` is used, it fails on GitLab CE,
3. If `IS_GITLAB_EE=0` it does work correctly, and disables EE,
4. If `IS_GITLAB_EE=1` is used on GitLab EE it effectively required
   to run on EE changes.

This changes to:

Make `IS_GITLAB_EE=` to donate the intent to disable or enable EE
sources, instead of requiring it, and makes the check consitent
in behaviour.

config/helpers/is_ee_env.js

@@ -3,7 +3,12 @@ const path = require('path');
 
 const ROOT_PATH = path.resolve(__dirname, '../..');
 
+// The `IS_GITLAB_EE` is always `string` or `nil`
+// Thus the nil or empty string will result
+// in using default value: true
+//
+// The behavior needs to be synchronised with
+// lib/gitlab.rb: Gitlab.ee?
 module.exports =
-  process.env.IS_GITLAB_EE !== undefined
-    ? JSON.parse(process.env.IS_GITLAB_EE)
-    : fs.existsSync(path.join(ROOT_PATH, 'ee'));
+  fs.existsSync(path.join(ROOT_PATH, 'ee', 'app', 'models', 'license.rb')) &&
+  (!process.env.IS_GITLAB_EE || JSON.parse(process.env.IS_GITLAB_EE));

lib/gitlab.rb

@@ -65,14 +65,18 @@ module Gitlab
 
   def self.ee?
     @is_ee ||=
-      if ENV['IS_GITLAB_EE'] && !ENV['IS_GITLAB_EE'].empty?
-        Gitlab::Utils.to_boolean(ENV['IS_GITLAB_EE'])
-      else
-        # We may use this method when the Rails environment is not loaded. This
-        # means that checking the presence of the License class could result in
-        # this method returning `false`, even for an EE installation.
-        root.join('ee/app/models/license.rb').exist?
-      end
+      # We use this method when the Rails environment is not loaded. This
+      # means that checking the presence of the License class could result in
+      # this method returning `false`, even for an EE installation.
+      #
+      # The `IS_GITLAB_EE` is always `string` or `nil`
+      # Thus the nil or empty string will result
+      # in using default value: true
+      #
+      # The behavior needs to be synchronised with
+      # config/helpers/is_ee_env.js
+      root.join('ee/app/models/license.rb').exist? &&
+        (ENV['IS_GITLAB_EE'].to_s.empty? || Gitlab::Utils.to_boolean(ENV['IS_GITLAB_EE']))
   end
 
   def self.ee