Side bar will show items without permission

[yottanami]

Faalis just tried to check permission of built-in menu items. and does not care about generated resources. and will show the header menu of resources without permission.

As I checked @model is nil in this line for none built-in items https://github.com/Yellowen/Faalis/blob/master/lib/faalis/dashboard/models/sidebar.rb#L12