More robust permission system

[alexpado]

For now, there's only 3 pseudo roles:

• Guest (guest = true and administrator = false on User)
• User (guest = false and administrator = false on User)
• Administrator (administator = true on User)

The goal would be to work on a more concrete role system to add support for "editor" users (allowing some /anime commands on discord, and later few access on the admin panel)