models.py: add TaskManager methods for roles
models.py: add TaskManager methods for roles

Three new methods: `instructors`, `learners`, `helpers`.
Each one filters default queryset for roles with specific name
("instructor", "learner", "helper" respectively).

shapiromatron/amy,swcarpentry/amy,vahtras/amy,wking/swc-amy,pbanaszkiewicz/amy,wking/swc-amy,shapiromatron/amy,wking/swc-amy,shapiromatron/amy,pbanaszkiewicz/amy,wking/swc-amy,swcarpentry/amy,vahtras/amy,swcarpentry/amy,vahtras/amy,pbanaszkiewicz/amy