Move username change decision into user model

app/controllers/profiles_controller.rb

@@ -51,7 +51,7 @@ class ProfilesController < ApplicationController
   end
   def update_username
-    if Gitlab.config.gitlab.username_changing_enabled
+    if @user.can_change_username?
       @user.update_attributes(username: params[:user][:username])
     end

app/models/user.rb

@@ -215,6 +215,10 @@ class User < ActiveRecord::Base
     keys.count == 0
   end
+  def can_change_username?
+    Gitlab.config.gitlab.username_changing_enabled
+  end
+
   def can_create_project?
     projects_limit > personal_projects.count
   end

app/views/profiles/account.html.haml

@@ -53,7 +53,7 @@
-- if Gitlab.config.gitlab.username_changing_enabled
+- if current_user.can_change_username?
   %fieldset.update-username
     %legend
       Username