Add option to disable username changing

Created by: raphendyr

When gitlab is used with ldap or pam for authentication, we might want to disable username changing.

We found out that you can reserve another users username before another user logs in first time and thus disabling the service from him.

app/controllers/profiles_controller.rb

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