Replace role notification system with direct ping

Currently the bot creates 1 role automatically per artist, which are assigned to users via the notifications command. This creates 2 Problem:

• A guild is only able to have a maximum of 250 roles
• Discord API rate limiting makes it so, that the creation of many roles at once takes time

To fix those issues, the goal is to replace the current role-oriented notification system with a user-id-oriented one.