Modify main node to register `/` commands

[ArmandBriere]

Is your feature request related to a problem? Please describe.
The ! is nice but the future of discord bot is in the slash commands

Describe the solution you'd like
We need to modify the main node to use slash command:

• Change discord.py to pycord https://github.com/Pycord-Development/pycord
• All command should have their function
• All command function must redirect treatment to cloud function
• All command function will have description
• If a command isn't active in Firestore, send back an ephemeral message back to user

[CmD0]

I'll start looking into implementing this.

[ArmandBriere]

#114