Change forum role granted to staff on enrollment

This applies to global staff (is_staff=True), not course staff.
Previously, staff were granted the Moderator role but not the Student
role upon enrolling in a course. If the Moderator role were later
revoked, then the user would have no role and be unable to post in the
forums, which is confusing for the user. edX staff indicated they would
prefer to not automatically receive the Moderator role, so the Student
role is granted instead. Note that staff will still be able to grant
themselves Moderator privileges through the instructor dashboard if
they wish.

JIRA: FOR-338