Refactor tasks_helper.py
Refactor tasks_helper.py

Moved shared code block into a separate method.  New code block takes in a
course ID and returns all student users actively enrolled in the course.

Stanford-Online/edx-platform,jbassen/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,caesar2164/edx-platform,jbassen/edx-platform,Stanford-Online/edx-platform,jbassen/edx-platform,caesar2164/edx-platform,jbassen/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,jbassen/edx-platform