tweak: permission check + code tidy up

[Spudgun]

A server check to ensure this event is able to be used by the client.

Coding 101: Never trust the client!

Also, a code clean-up in preparation for some features I'll be adding to the menu as discussed with @GhzGarage

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[tom-osborne]

@Spudgun I tested this and both the vehicle list and dealer list do not show any results. Fresh QBCore server build. Standard qb-adminmenu works fine, switching to your fork, breaks those menus. Can you take a look please?

Spawn vehicle list:

Dealer list: