Have a list of ignored roles for introduction reminders (and purge)

[Thatsmusic99]

Following today's nuclear fallout, it's become noticed that certain roles are not blacklisted from introduction automations (e.g. reminders), such as applicants, staff and other externals (we would need to make a role for this ourselves). It would be nice to have a feature in place to mark users as exempt from reminders, similar to how guests are.

I'd like to hold off implementing such a feature in the production bot until this Wednesday, so we can discuss this a bit more in depth and other committee can give their thoughts. :)