Fix admin permissions

[Nipis]

Fixed the admin panel permission system.

This fixes the following issues:

• Operators and Super Operators were unable to view the main page of the admin panel.
• Operators and Super Operators were unable to access areas they have permissions to. They always saw the message "You need special permissions to view this page."
• Any user could view/edit another top-level user.
• The level of an own account could be changed.
• The level of another user could be changed to a higher level than the user themselves.
• New accounts could be created from the panel with a level equal to or higher than the user's own. (Even though they can edit the account later to give it the same level, it's better this way to avoid errors when creating an account.)
• Top levels are no longer displayed in the combobox when editing a user.
• Own and top levels are no longer displayed in the combobox when creating a new user.