feature(delagent): improve permission checking

• pass the variables user_id and user_perm to all functions where they are needed
• extract the permission checks into three functions (for uploads, folders, licenses)
• start adding the permission checks into the correct places (not done yet, not tested yet)
• move admin_folder_delete.php to delagent/ui
• general rewriting of the delagent

partly based on work by @shaheemazmalmmd