On_member_join: grant configurable role

[maartenberg]

• Track list of roles per server in database, settable by Admins
• On member join, look up server and grant roles if possible and roles are set

User-facing:

• !adddefaultrole [@role | role_id], admin (not role but Server admin) only
• !listdefaultroles, lists roles
• !deldefaultrole [@role | role_id], same

[maartenberg]

https://discordpy.readthedocs.io/en/latest/api.html#discord.on_member_join

https://discordpy.readthedocs.io/en/latest/api.html#discord.Client.add_roles