Add option to disable username changing

app/controllers/profiles_controller.rb

@@ -51,7 +51,9 @@ class ProfilesController < ApplicationController
   end
 
   def update_username
-    @user.update_attributes(username: params[:user][:username])
+    if Gitlab.config.gitlab.username_changing_enabled
+      @user.update_attributes(username: params[:user][:username])
+    end
 
     respond_to do |format|
       format.js

app/views/profiles/account.html.haml

@@ -53,28 +53,29 @@
 
 
 
-%fieldset.update-username
-  %legend
-    Username
-    %small.cred.right
-      Changing your username can have unintended side effects!
-  = form_for @user, url: update_username_profile_path,  method: :put, remote: true do |f|
-    .padded
-      = f.label :username
-      .input
-        = f.text_field :username, required: true
-         
-        %span.loading-gif.hide= image_tag "ajax_loader.gif"
-        %span.update-success.cgreen.hide
-          %i.icon-ok
-          Saved
-        %span.update-failed.cred.hide
-          %i.icon-remove
-          Failed
-        %ul.cred
-          %li It will change web url for personal projects.
-          %li It will change the git path to repositories for personal projects.
-      .input
-        = f.submit 'Save username', class: "btn save-btn"
+- if Gitlab.config.gitlab.username_changing_enabled
+  %fieldset.update-username
+    %legend
+      Username
+      %small.cred.right
+        Changing your username can have unintended side effects!
+    = form_for @user, url: update_username_profile_path,  method: :put, remote: true do |f|
+      .padded
+        = f.label :username
+        .input
+          = f.text_field :username, required: true
+           
+          %span.loading-gif.hide= image_tag "ajax_loader.gif"
+          %span.update-success.cgreen.hide
+            %i.icon-ok
+            Saved
+          %span.update-failed.cred.hide
+            %i.icon-remove
+            Failed
+          %ul.cred
+            %li It will change web url for personal projects.
+            %li It will change the git path to repositories for personal projects.
+        .input
+          = f.submit 'Save username', class: "btn save-btn"