JIRA: FOR-398, FOR-399

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

switch to using get() rather than get_or_create(). Also add a test case with a different casing on the courseId

define a unseeding forums permissions and call into it from delete_course

define a unseeding forums permissions and call into it from delete_course

Features coming down the pipe will want to be able to:
* Refer to enrollments before they are actually activated (approval step).
* See what courses a user used to be enrolled in for when they re-enroll in
  the same course, or a different run of that course.
* Have different "modes" of enrolling in a course, representing things like
  honor certificate enrollment, auditing (no certs), etc.

This change adds an is_active flag and mode (with default being "honor").
The commit is only as large as it is because many parts of the codebase were
manipulating enrollments by adding and removing CourseEnrollment objects
directly. It was necessary to create classmethods on CourseEnrollment to
encapsulate this functionality and then port everything over to using them.

The migration to add columns has been tested on a prod replica, and seems to be
fine for running on a live system with single digit millions of rows of
enrollments.

add a migrations to the new django_comment_common to keep things a bit more consistent with our normal useage.

update comment models.py to not have the dependency on get_course_by_id (which is in LMS). Also explicitly set the db_table name because django auto generates table prefixes according to the djangoapp name

move some files from lms/djangoapps/django_comment_client to common/djangoapps/django_comment_common