fix: do not issue certs to suspended users

[rhell4]

Currently there is a scenario where a suspended user could be issued a cert.

1. Student is enrolled in course
2. Student is later suspended
3. Teacher manually removes the issued cert for the student (or the student didn't have a cert issues to them yet)
4. Issue certificates task runs
5. Suspended student is issued a cert
6. If "Email students" was turned on the suspended student receives an email, leading to confusion

I think just a simple check of the suspended flag and skipping that user should suffice. I've also adjusted the unit test to cover this.