Sidebar for modules in admin

Update info:

• You should migrate your database to add preferences field to users table php artisan migrate
• Manually add icon_field (varchar nullable) to menulinks table.

Changelog:

• Admin sidebar
• Admin menus managed by Menus module
• New facade for Pages module
• Added icon_class field to menulinks table
• Added preferences field in user table
• Added method to get and set current user preferences
• Menu facade is now plural (Menus)
• Bugs