Add permissions system

Add a permissions/roles system. Some commands, which should only available to the staff of the event, are now executed only if the user has enough permissions.

Currently implemented 2 roles (Admin and Staff), which allows to have a hierarchical system.

Here follows the list of roles:

• Admin: Has maximum access and can perform any action.
• Staff: Has more access than the normal user but can't perform all comands that an Admin can.

The actual list of commands and the permission needed to execute it are as follows:

Command	Description	Permission
/criar-equipa	Create new team	Admin
/entrar	Join team	Any user (without team)
/saldo	View team balance	Any user (with team)
/compra	Buys an item	Any user (with team)
/movimentos	List last transactions	Any user (with team)
/ver-equipas	List all teams	Admin, Staff
/ver-equipas-registo	List all registered teams	Admin, Staff
/detalhes-equipa	View one team details	Admin, Staff
/detalhes	View one participant details	Admin, Staff