Add db index for active user display.

CHANGELOG

+v 6.9.0
+  - View active users in the admin dashboard
+
 v 6.8.0
   - Ability to at mention users that are participating in issue and merge req. discussion
   - Enabled GZip Compression for assets in example Nginx, make sure that Nginx is compiled with --with-http_gzip_static_module flag (this is default in Ubuntu)

db/migrate/20140416185734_index_on_current_sign_in_at.rb

+class IndexOnCurrentSignInAt < ActiveRecord::Migration
+  def change
+    add_index :users, :current_sign_in_at
+  end
+end

db/schema.rb

@@ -332,6 +332,7 @@ ActiveRecord::Schema.define(version: 20140414131055) do
   add_index "users", ["admin"], name: "index_users_on_admin", using: :btree
   add_index "users", ["authentication_token"], name: "index_users_on_authentication_token", unique: true, using: :btree
   add_index "users", ["confirmation_token"], name: "index_users_on_confirmation_token", unique: true, using: :btree
+  add_index "users", ["current_sign_in_at"], name: "index_users_on_current_sign_in_at", using: :btree
   add_index "users", ["email"], name: "index_users_on_email", unique: true, using: :btree
   add_index "users", ["extern_uid", "provider"], name: "index_users_on_extern_uid_and_provider", unique: true, using: :btree
   add_index "users", ["name"], name: "index_users_on_name", using: :btree