Assign onsite/remote volunteer and organizer roles

[NMertsch]

Upon registration, the bot assign Discord roles.

In 2024, there were multiple scenarios where roles had to be assigned manually, and this should be avoided as much as feasible next year:

• Organizers and Volunteers had the same ticket type, so the "Organisers" role had to be assigned manually to all organizers.
• There was an overlap between speakers, volunteers, organizers, and sponsors. Depending on the ticket, the other roles were missing (e.g. volunteers with speaker ticket don't get the volunteer role, speakers with sponsor ticket don't get the speaker role).
• Volunteers have a volunteer ticket, but the roles "Onsite Volunteers" and "Remote Volunteers" had to be assigned manually.

Some changes Pretix with the Pretix ticket type configuration might be required to implement this.